Disgaea/WMG

Makai Kingdom is the meta-verse for all Nippon Ichi games

 * The existence of the Sacred Tome, a reality warping book that can create anything as long as you pay enough Mana, seems to suggest this.

The main characters are all in Hell.
...wait a minute.
 * Crazy talk.

The setting takes place an unimaginably long time after the Blood War ended.
An infinite number of combinations of Lawful Evil and Chaotic Evil Hells battling for supremacy. It was to last forever; but nothing is forever. An infinite number of combinations will eventually lead to new ideas. Ideals clashed, destroyed each other, fused, fizzled out; evolved. There are still some of the greater evils left; and many of the Overlords are still fighting each other for supremacy but nothing as organized as the Baatezu or the Tanar'ri. The Blood War ended; not with a bang, not with a whimper, the eternal Wheel of Time merely lead to them becoming their own worlds.
 * Makai Kingdom was the last really big flare-up, but that situation was taken care of.
 * Of the old guard, only Baal remains.
 * The Disgaea Netherworld, aka Makai, was formerly known as the Abyss. It currently barely resembles it former self now.

Flonne was evil all along.

 * In the anime, at least, Flonne and Vulcanus are the only angels that enter the Netherworld and survive long enough to leave. And Vulcanus was a villain, of the Well-Intentioned Extremist variety.
 * And that's besides the "reverse-Noble Demon" aspects -- cheating Laharl out of a reward for saving her life, breaking Robot Buddy Thursday after he's about to reveal her foul language ...
 * Fallen Angel Flonne is little more than a Palette Swap of her angel form.
 * Not entirely accurate, she has demon wings and tail in 2nd form, and her blouse is open, shwoing her in a red...one piece bathing suit? Also, she probably chucked the pendant after becoming Fallen, as she only needed it to survive in the demon world, and now that she's fallen, she doesn't need it(Remember, Clerics are described as 'possibly' fallen angels).
 * Wouldn't her own pendant burn her? Also, would she not respawn as a Prinny upon death? (since Laharl appears to be able to, unless that's because he's half-human, which would contradict a theory below)
 * She could easily have been lying about what sets off the pendant. As for the Prinny thing, it's obvious that she's too cute to become a Prinny.
 * Except that it actually reacted to Laharl and Vulcanus's bad thoughts, and a Prinny Flonne would be amusing.
 * Do we know that? That's what everyone thinks sets it off, but maybe it's the simple fact that someone other than Flonne is holding it. Remember, nobody else in the series even tries holding the pendant, and when Laharl does handle it without being burned, Flonne's a flower. A Prinny Flonne, though... yeah, it would be amusing, I'll give you that.
 * Mid-Boss and Larharl (end of game) held it without being burnt.
 * OK, so someone else holding it isn't the trigger, but maybe it only hurts the holder when Flonne wants it to. When Laharl first holds the pendant, Flonne doesn't like him much, so she could easily have wanted to hurt him then. When Midboss holds it, he's just helped Flonne, so she'd probably not want anything bad to happen to him then. By the time Vulcanus holds it, Flonne's already at least suspicious of him. And when Laharl holds it without being hurt, as mentioned before, Flonne's a flower, and therefore incapable of making the decision. (Or, if she is capable, she actually likes him by then.) Also note that Flonne is close to the pendant every time someone else is holding it.
 * If she was able to activate it, it would have burned Mid-Boss when she realized that he had it.
 * She wasn't suspicious of him. She didn't even know he was there. And before you mention the feather they found, she just mentioned that it was that of a high ranking angel and nothing more.
 * Why would she activate it then? Who said she's supposed to be Chaotic Evil?
 * It didn't burn Mid-Boss because his motives are actually good, Batmaning Laharl into becoming a Noble Demon. Likewise by the time Laharl is a Noble Demon he doesn't get burned, but when he was still evil, he did. The pendant appears to work on motives, not actions.
 * Yeah, if that's how it actually works. But if Flonne is evil, that's not how it works, ergo, that point is moot. Of course, it could just be that the pendant does indeed burn anyone who is evil, but Flonne's immune to it. Or maybe it only burns evil males, and Flonne just omitted that part? After all, everyone the pendant did burn was male.
 * I don't see how Flonne becoming evil would change how the pendant works. Besides, the pendant given to her by Lamington originally.
 * The guess is not that Flonne became evil, it's that she was already evil. That being the case, we have every reason to believe that the pendant doesn't work the way that Evil!Flonne says it does, hence the extra WMG over why it would've done what was observed.
 * Except it was the Seraph who gave her the pendant, wasn't it? She didn't create it, she was told how it only worked for those with kind-hearts.
 * Did we actually see him give her the pendant? If not, easy explanation -- again, she lied.
 * Flonne? The Love Freak Trope Namer? Evil? HAHAHAHAHAHAHAHAHAHAHOHEHAHAHA* gasp* HAHAHAHAHAHAHAHHAHAhahahahehe* pant* hoooooo... You're kidding me, right?.
 * Love Makes You Evil, you know.
 * See the OVA's when .   more or less tortures the Prinnies by having them do good deeds non-stop, practically killing them from overexhaustion.
 * As opposed to sending them on suicide missions?

Angels are just a subset of Demons who decided to make their world more pleasant.
They also get Prinnies, and Laharl is able to create them as troops if he satisfies certain conditions. Thus Flonne "falling" was just a case of simple transmigration. This would also explain the above example; the other Angels other than Lamington are either insane or clinically depressed, due to not quite being what they think they are. Flonne is just "evil" enough not to be bothered, and happy no matter what she calls herself. Actually how would the angels survive in the underworld?.....
 * If you meet the requirements, Larharl is also able to create Angels(!) and Humans (EDF).
 * Well, it's All There in the Manual, but apparently, there's been a angelic influence in the Netherworld for some time now. The Succubi monsters and Healers are all descended from fallen angels. Presumably, the angels you can get either are fallen, or come with their own protective amulets.
 * The angels you get to create are all full-fledged angels, no need for the amulets.
 * This is true in the Christian religions, the demons come from the rebel angels who followed Lucifer in his war against God and were sent to Hell to stay there until they repent. It's not hard to believe this may be the inspiration.
 * In this case, it's a complete inversion of the Biblical myth.

Laharl's Netherworld is one of many that's become taken over by "human" Demons and their descendants.
Humans can become Demon Lords and Demon Overlords if they kill enough non-humanoid demons. In many of the Netherworlds, Laharl's included, it's the Humanoid Demons who form a dynasty; and Laharl's mother was a human, yet nobody calls him "half-human." Makai Kingdom characters suffered some cognitive dissonance over this; they apparently justify it as Demon being how you act, not your origins.
 * It's true that the half-human issue doesn't seem so big in-game, but there are certain sects in the Netherworld(Laharl's relatives, in fact) that are very discriminatory towards humans and consider Laharl's blood to be 'soiled'.
 * I don't think the monster/humanoid thing is well thought out. Its interesting that Prier actually BECAME a demon by killing lots of demons.
 * It's a Japanese mythology thing. One who kills 1000 Yokai becomes a Yokai.
 * It should also be noted that none of the demons in the Dark World stages of La Pucelle were humanoid (Dark Warriors might be)

Chou Maou Pearl isn't just Baal, he's Legion.
He's Legion who has possessed Baal. As we learned in Makai Kingdom, "Baal" just possesses another body whenever his current one is destroyed, and this usually includes other demons; he has a special affinity to Prinnies, though. Then in Disgaea 2 we discover he can possess more than one body at a time. For he is many.
 * Possible, or he could just be using a variant of the Doppleganger spell of the Clone Geo Panel.

The Disgaea series takes place in an infinitly more idealistic version of the Warhammer 40000 Universe

 * Let's look at the facts:
 * Both games run on Rule of Cool to an extent
 * The netherworld and the warp are both in space.
 * Humanity is in some sort of spaceborne phase.
 * Succubi and Nekomata are absolute Fetish Fuel...
 * And with a level cap of 9999, there is always More Dakka.
 * This world mean that the chaos gods are simply demons who are level 9999(or higher?).
 * We definitely know that there is a tranformation past "Demon Overlord." Baal has accomplished this; and Zenon barely escaped that fate. (It's more than just a title change to Tyrant or Master).
 * Or conversely, Disgaea IS Warhammer 40k, filtered through the perspective of Chaos Demons. What reason would they have to see their world as anything but delightful and adorable?

"Demonicity" sprang from a single source that shattered near the beginning of time. Demons are shards of that power; held in check by the free will of their souls.
Like a Black Magic big bang, pieces of this (Let's call it the Angra Manyu) spread across the multiverse; coalescing into various Powers and monsters. The "monstrous" demons are likely to be original models. But as humans became stronger, they started to defeat demons and absorb the trait themselves; creating humanoid demons like the main cast of Disgaea and Makai Kingdom. Demons can get more power and advance their titles by defeating others; but as they do so they run the risk of the "original will" taking them over. Baal is/are the largest chunck(s) leftover from the beginning; lesser chunks such as Sulphur from Phantom Brave also exist.

Arcana Heart takes place in the Nippon Ichi multiverse

 * Half demon Lilica's idea of "evil" is skipping school and rollerblading where she shouldn't, sound familiar?
 * Laharl is also ; does this mean that Lilica is Laharl's sister?
 * Think about this. Lilica's mother is a human and Lilica's father is a demon. Laharl's mother is a and Laharl's father is King Kricheveskoy the demon overlord.
 * Oh. My. God.

Similarly the Makai of Touhou Project is also part of the Nippon Ichi multiverse

 * Mostly if you realize that the "Netherworld" of Disgaea is called "Makai" in Japan...and there's more than one!
 * Meaning either demons don't bother with it for some reason, or it was abandoned + sealed off after Reimu, Marisa, Mima and Yuuka were through with it (and Sanae, depending on which of the UFO endings is canon)

Princess Sapphire is a tiefling.
IE, a human with some demon blood in her lineage. Hence her unbelievable strength, bizarre star eyes, and tendency towards overthetop bloodthirsty acts belied with good-naturedness.
 * Disgaea Tieflings are different than Dungeons and Dragons Tieflings.

Princess Sapphire is part-fey.

 * Not any specific type mind you, but she has some traits common of The Fair Folk: Beautiful, otherworldly apperance... and amoral good-natured bloodlust.

Princess Sapphire is part-angel.
Firstly, Sapphire is adept at healing and bows, just like angels, and her personality is sweet and innocent (startlingly so, all things considered), much like Flonne is. In addition, her overwhelming strength can be attributed to the angel blood in her veins- after all, the power of an angel is the same as the power of a demon! Because she didn't grow up under the teachings of Celestia, her heart developed more along the lines of a demon, which explains her affinity for axes and violent tendencies. Design wise, Sapphire has a blue and white colour scheme, which thematically matches the tier 1 angel (especially the fluffy parts of her dress), so that further supports this theory.

Mid-Boss is everywhere.
Prism Red's voice in the English version of Disgaea 3 sounds familiar, doesn't it? It may not be the same Prism Red who was in the first two games. After all, one of the homeroom teachers in the game is a zombie, claiming to be the "original" Prism Red, so we know that there has been more than one set of Prism Rangers. You might be saying the fact that Mid-Boss is now a downloadable character disproves any theories that he might actually be the new Prism Red. I say it's just a clever ruse to throw the scent off the trail. I mean, he has shown in at least two of his special moves that he can duplicate himself, so who's to say he can't create at least one sentient doppelganger?

And for that matter, with the above theory that Baal is Legion... have you ever seen Mid-Boss and Baal in the same scene? It has been mentioned multiple times that they have fought at least once, but the fight itself is never shown, only described.
 * It shouldn't matter whether they're both downloadable or not. I mean, you can download both Kurtises...obviously causality is trivial in the Netherworlds.
 * At least where travel from one to another is concerned.
 * Mid-Boss is just really really fast. He trains with 100-ton boxers, you know. And.

Adell

 * It does say
 * Except Rozalin
 * Except Rozalin

Dark Elves are Demons.
We're talking old school Norse/Germanic legend here. There's a lot of brands of demons in the Nippon Ichi verse; they're not all the same species. The magic-using, long-lived, Pointed Ears, bizarre morality'ed version are Dark Elves. Age appearance is either dependent on self-image or possibly how much human blood is in their lineage.

Kyoko and Asuka have to obey Raspberyl.
Raspberyl is likely a Demon Noble; so she has two servants whose job is absolute. Obey her every whim. Hence them calling her "My Lady/Ojou-sama". This is not to say they haven't been won over by her Noble Demon stance on things, but this is how it started.

Raspberyl is a Half-Humanoid Hybrid.
Her parents are a Monster-class Demon and a Humanoid-class Demon. Hence her being a Monster-class, yet looking humanoid; and her having horns. She ages slower than other humanoid Demons; hence her loli state. This set her apart enough that she figured "Screw it, if I'm gonna be different; than I'm going to be different all the way." So she became a delinquent. Her friendship with Kyoko and Asuka stems from them being rebels in the sense of their friendship together. Ninja and Samurai have distinct cultures All There in the Manual; its likely they have a rivalry.
 * Specifically, she is part Succubus. She has all of the traits: Pinkish hair and eyes, the similarly shaped tail and wings. And oddly enough, her habit of "Shipping" things and delinquency could also come from part of her Succubi parentage. After all, Succubi like being bad, Raspberyl just expresses that urge differently.
 * Yes, Raspberyl does occasionally ship Kyoko and Asuka together as a couple. Her Dark Hero Days encounter shows she loves forbidden love. Whether or not the two are a couple; they're used it her doing that.

Adell
A central concept in Disgaea 3 is that Almaz, having lost his title of "hero", is transforming into a demon despite being originally human -- being given a title of "demon in training" is enough to trigger a transformation from human to demon. On the flip side, this could mean that,
 * Tell me where in the Great Book O' Tropes that
 * I always thought his title was an intentional joke. Turns out he's not just a Demon Hunter (a hunter of demons), he's also a Demon Hunter (a hunter who is also a demon).
 * I always thought his title was an intentional joke. Turns out he's not just a Demon Hunter (a hunter of demons), he's also a Demon Hunter (a hunter who is also a demon).

According to Something Awful Forums, Aurum's the protagonist of one of those games that when the player dies, the king revives him or her and takes half of the gold.

 * Aurum:
 * Nippon Ichi has elaborated.
 * Nippon Ichi has elaborated.

Demons look as old physically as they are mentally.
Note that despite being Really Seven Hundred Years Old, Laharl and Etna still act a lot like little kids. Likewise, Mao and Beryl are teenagers. It takes mental maturity for a demon to reach adulthood, and the kind of issues the protagonists often have are counterproductive to this.
 * This is actually supported in universe. Consider that Etna had been serving Krichevskoy since before Laharl was born. They're only roughly a hundred years apart. Etna Mode's story about him taking her in clearly proves she wasn't an infant at that point. So either they've got some looooong aging plateaus, or they don't age at a steady rate. That also explains Mid-Boss, to an extent.

Baal is Gozer
They're both ancient, highly feared, supernatural beings who change their forms each time they emerge, and both have transformed into something that looks completely harmless, but is really even more dangerous than the previous form. It's not too hard to imagine that Baal sometimes enters the human world under a different name. There is the question of why Vinz Clortho and Zuul don't follow him in the Netherworld, but given the fact that they're statues until it's time to summon "Gozer", they probably just remain as statues while Baal roams the Netherworld.
 * They're probably "Entei" demon-class like the ones you fight in the first game before you can get to Baal. Yeah, they took the form of dogs in the movie; but that's because Ivo Shandor made those statues that way.

Adell has a birth defect .
Adell chooses not to think; but not that much.
 * Confirmed by Word of God... maybe...so... not confirmed?

The events from La Pucelle are directly responsible for the entire plot of Disgaea
King Kricheveskoy had sealed off Baal in the human world of La Pucelle. However, during that time, Priere and co crashed into the Dark Shrine and proceeded to kick the living daylights out of the original Baal, breaking the seal to get in, thus allowing him to leave the seal and switch bodies. That set off Kricheveskoy's fight against Baal, which eventually killed him, but not before resealing the new Baal, possibly locking up more entries to that same world, such as Priere's Netherworld and Marjoly's Castle (since they all belong in the same multiverse).
 * Prier made him angrier by "purifying him" and then selling him to slavery. Baal very, very quickly made it up the ranks.
 * Revisiting this theory after the new endings of La Pucelle Ragnarok are declared. "Maiden of Darkness" Prier may have led Seraph Lamington to step up his plan regarding demons and angels; since she proved that it was possible to be good and a demon.

The Heroic Prinny is a piece of Baal.
Supposedly the singular Prinny is really a collective 1000. Yet the one that's "in charge" has the same personality; always. Whenever one falls, the soul just goes to the next one. The Scarf is really a Restraining Bolt that Etna came up with with the side effect of stopping Sir Sweet. Seriously, do you really think Sir Sweet was such a threat Etna had to sneak attack him? No, her actual task was keeping the Baal piece occupied. Why would Etna do this? Baal killed King Krichevskoy; she'd hate Baal with a passion. Because she has a plan. NOTHING kills Baal. He always comes back; his soul possessing some other body. But...what if, he were redeemed? If he paid the Prinny price of the Netherworld, and his soul reformatted by the Red Moon; he would be absorbed back into the cycle of life; his powers gone. And results mean far more to Etna.
 * Yes, Baal is a Bonus Boss if one replays the stage where you fight . He has more than one body going around; and really that was surprisingly weak for Baal.
 * Keep in mind, Prinny was an Alternate Universe; so everyone else is still fighting Baal.
 * The fact that the Prinny in charge always comments on used to being a "Dirty Cop" in life cements it. All 1000 were Dirty Cops? I think not. It's Fake Memories Etna placed on him/them.

Veldime didn't just automatically snap back when the Curse ended.
There was demon traffic there even before the Curse; and during there were plenty of beings who traveled to and from it. It's also possibly the Snap Back that happened to Hanako and Taro happened to most of the demonic children born during the Curse. And even if most of the beings did turn human, there's a serious network of beliefs and traditions the classes established; Healers, Fighters, Thieves all had guilds. "And it all went away" is unlikely.
 * Not that this is a bad thing. Not all demons are evil; Veldime just became a hybrid society afterwards. Adell going to the Academy of Evil because things had gotten "boring" is a good sign.
 * It was a hybrid world before; as the Snow Clan ninja demons were both native and "true demons."
 * Arguably Word of God, if you accept The World of Disgaea 2 as canon; page 47 tells us while the transformed people changed back, it's otherwise unchanged. In fact, once a world is transformed into a netherworld, it's stuck that way. Barring, presumably, a trigger-happy moron destroying it utterly.

Gordon and Jennifer aren't human anymore.
There's actually more than one theory on this one, but it's mostly based on the curious fact that they're of the Monster Class in Disgaea 3 Absence of Justice who can Magichange:
 * 1) Exposure to the Netherworlds and being Vassals to Laharl had a nativizing effect. Undergoing the Transmigration/Reincarnation process to strengthen bodies would accelerate this effect.
 * 2) Travelling too close to Veldime in Disgaea 2: Cursed Memories inflicted them with part of the Curse. Gordon was a Monster class there too and Magichangable.
 * 3) They aren't the original Captain Gordon and Jennifer. Kurtis's "teaching" the Kurtis Copies how to have a soul radically accelerated the EDF's Robot Defender program; and Gordon's and Jennifer's information was added to the roster. The new improved Neural interface also gave DLC Thursday the ability to Transmigrate.

Prinnies that were generally good in their past life are 'rewarded' in their prinny life by being distinguishable from their murderous kin.
Simply put, Prinnies who were decent people in their past life and only missed being reincarnated because of ether their dying actions or by them not being totally virtuous  are reincarnated into a Prinny but instead of being made into the standard blue Prinny they instead take on some degree of features from their past life, typically having the voice they had in their previous life, becoming the color of hair they were in their past life and taking on another distinguishable feature. They may also be able to not end their sentences with the standard dood/-ssu.
 * Could also be attributed to strength of character.

Disgaea 2: Cursed Memories takes place after the main plot of Disgaea 3 Absence of Justice
But before the "Teacher Crisis" and DLC Raspberyl plot. Mao goofed around long enough for Almaz and Sapphire to get married, afterall. It's doubtful they eloped, and royal marriages take time. This only happens at the end of Disgaea 3 Absence of Justice. Note, all of the DLC characters in Disgaea 3 Absence of Justice are only available after the main plot; while Mao petitions as Dean to the Dark Council at the beginning of Disgaea 2: Dark Hero Days.

Adell is a Malakim
Hmm... because of his oaths? The chains  are invisible.

Adell
His parents mention that  So presumably they were   and he's been that his whole life.

The only thing that separates Humanoid-class characters from Monster-class characters is the ability and/or willingness to lift and grip objects.
Gordon and Jennifer may be Monster-class characters in Disgaea 2 and 3 because the objects and weapons in Veldime and Evil Academy might just be too heavy for them. Same deal with Raspberyl, who uses her book for all of her attacks. Others, such as Mid-Boss, Vulcanus, and Aurum might be able to, but simply choose not to for various reasons. For example, Longinus is described as Krichevskoy's old spear, and Meanwhile,  is constantly holding a pair of swords in his One-Winged Angel form, so maybe he's just too attached to those particular swords to equip any other weapon.
 * And once you give a Prinny a fancy red scarf, it can suddenly pick up and throw things.
 * Prinnies can already do that. Infinite Bag Of Holding around their waists, remember? They have so much crap in there that them picking up and carrying other stuff is pretty pointless in the end - anything they may pick up from the field barring enemies they probably already have in those bags... Besides, Prinnies explode when thrown, so they have reasons to not get too close to enemies capable of grabbing them.
 * According to the Disgaea 2 Art Book, monster-class characters fight using their own strength and abilities (and if any do use weapons, such as the Orcs, theirs are rather old), the monster-class weapons only used to strengthen their powers. Humanoid-types (most of them, anyway) have to use weapons and their specific skills to make up for their lack of innate abilities (and stealing their weapon leaves them a sitting duck).

Hell and Heaven are entirely different from the Netherworld and Celestia
This would explain why the demons refer to Hell as if it were a different place, and why God (who is mentioned indirectly by the angels, and in Etna Mode) is apparently absent from Celestia.
 * Perhaps the Demons and Angels go to Hell and Heaven when they die? Humans seem to anyway, so there wouldn't be a point for them to go there.
 * I find it far more likely Heaven/Hell are either an alternate Celestia/Netherworld, a particular region inside Celestia/Netherworld, A artifact of human speech passed down to demons, or more of a Buddhist-style state of being. It also may be God is actually a transcendent deity or myth to angels as well as humans, or simply doesn't have a real body, or lives in an alternate Celestia. Also I think there is a reason to think Celestia and The Netherworld exist as parallels to human belief in heaven and hell, but are clearly different in reality.
 * Makai Kingdom has Seedle, lord of the Underworld "you know, where the dead people are". So there might be some version of Purgatory.
 * It's everything but outright stated that Heaven and Hell are separate from Celestia and the Netherworld. For instance, in the first Disgaea, after Kurtis dies, he reincarnates as a Prinny in Celestia, and says he needs to make up for his mistakes in life before he can go to Heaven to join his family in paradise. Additionally the Rasberyl DLC, when they're going to Celestia, Sapphire asks if they have to die to get there (Then asks for volunteers). Master Big Star assures them that Celestia "isn't what they're thinking of".  Thus, you could say that Celestia and the Netherworld are both forms of Purgatory that Prinnies must endure until the Red Moon, where they can go to heaven properly. There is no 'endless cycle'. Being a prinny is a chance to make up for your mistakes as a human.
 * In Seedle's case, his Netherworld probably acts as your typical Purgatory, without sinners reincarnating as Prinnies. Maybe as Phantoms? It's possible that people don't necessarily have to be Prinnies to atone. See: Disgaea 2, 'Reincarnate to Atone for Sins/into a Prinny'. As for demons and angels, well, at one point in Makai Kingdom it's stated that if an Overlord dies, there's nothing left. Possibly this could mean if they die outside of their Netherworld, or without any mana left, then a demon is gone for good, but otherwise they may well reincarnate somewhere else or even go to Heaven or Hell depending on what they were like in life.

Overlord Zenon became a being like Baal at the end of Disgaea 2.
If a mere human becomes an Overlord by killing 1000 demons, what would happen to a demon who kills 1000 overlords? Or an Overlord who kills that many Overlords? The fake Zenon was merely the last Overlord to be killed before Zenon fully became an Eldritch Abomination akin to Baal. Think about it. When Baal dies he jumps to the nearest body available, and presumably the strongest if at all possible. What happens when Zenon(in Rozalin) is killed by Adell in the Worst Ending? She jumps into Adells body, continuing to live. Of course this only makes Adells victory via smooch all the more Glorious.
 * This might be what the legendary title of Supreme Overlord is really about.
 * Didn't Laharl have a few lines in the original about something above Supreme Overlord? Wiki Magic?

Humans, Angels, and humanoid Demons are the same thing.
The difference being slight mutations and slightly different powers which is more about the world they live in then anything else. After all, humans and angels both have turned into demons, and perhaps the reverse is true.

Champloo is
This is actually confirmed in the Mid-Boss DLC. Mid-Boss was

(Real) Zenon is a female actually.
But fake Zenon is a male, of course.

Laharl was born to constantly kick Baal's ass
I'd go as far as saying he has "Baal senses". Every time Laharl shows up, Baal always ends up as the bonus boss. This happens in Disgaea Hour of Darkness, Phantom Brave, Makai Kingdom, Disgaea 2: Cursed Memories, Disgaea 3 Absence of Justice, and Prinny. And then in the Updated Rerelease of La Pucelle, he subverts time and space to get him, Etna, and Flonne into the game just so that he can kick Baal's ass. Yes, it could be his popularity bringing him back all the time, but then why do you think he doesn't show up in any of the crossover games? Only Etna and a Prinny show up in Cross Edge and the Japanese-only Trinity Universe has only Etna, Flonne, and a Prinny. It's pretty obvious why: Baal didn't show up, thus the crossover games were not worth his time. Yes, there's Soul Nomad and The World Eaters, but he didn't stick around for long. He just complained about Revya taking the title of Overlord lightly before quickly leaving him/her and Gig. And remember in Disgaea 3 when Laharl said that before he went to the Evil Academy he was fighting Baal until he somehow escaped? Where do you think he was fighting him? It's possible that Baal was planning on showing up in Soul Nomad to challenge Revya and Gig, but upon seeing Laharl again, he ran off into another game because he was so tired of fighting Laharl all the time. Laharl, with his Baal senses, noticed this and chased him to Evil Academy. Screen time might be Laharl's secondary objective, but it's pretty damn clear that his primary reason for his constant appearances in most games after his debut in Disgaea Hour of Darkness is to kick Baal's ass wherever he goes.
 * This might actually not really be a matter of "he was born to do it", but of revenge; Baal is the one who really killed Krichevskoy, after all. Hello. My Name Is Overlord Laharl. You Killed My Father. Prepare to Die.
 * I have thought that, but Baal has appeared so many times that I'm starting to think Laharl wants to do it for kicks rather than just avenge Krichevskoy. Then again, he is aware that Baal constantly transfers into another body once he's defeated so maybe he's constantly fighting Baal since he knows he can never truly be destroyed.
 * Laharl himself makes claims to the effect that it's all about showing the universe - and everything in it - who's boss.
 * Yeah, but if that was true, why doesn't he show up in crossover universes? Again, he's not in Cross Edge and Trinity Universe, and he didn't stick around too long in Soul Nomad (And in that particular he case, it was only in the Demon Path).
 * Has anyone considered that it may be the other way around? That Baal shows up solely wherever Laharl goes because nobody else is worth his time and he's looking for someone strong to throw down with in a massive fight? After all, Baal can't really die, so he can afford to seek out the strongest being he can find so he'll have a good time, and he sure seems to like causing as much chaos as possible...
 * Seems Disgaea 4: A Promise Unforgotten has debunked this bit, seeing as how Baal's nowhere to be seen in that game... and considering that in Zettai Hero Project, Valvolga from Makai Kingdom is stated to have been the one to off Baal permanently.
 * Not so fast! Baal is returning as DLC! So among all the games Laharl appeared in, the only game that didn't have Baal was Zettai Hero Project.

Succubi aren't constantly Gainaxing to seduce, but because they are on a permanent sugar rush.
I know that this is a bit of stretch, but I have made some observations:
 * They gainax even when nobody is around for them to seduce.
 * They always jump up and down in a manner that is characteristic of a hyperactive child.
 * Their facial and body expression both in the sprites and in the portraits looks more enthusiastic and wide-eyed rather than sullen and/or seductive.

On these observations, there's the possibility that they are merely a group of Innocent Fanservice Girls. Or more likely, that they are not always so blatant of their sexiness as it seems. Rather, this phenomenon of permanent Gainaxing is just a simple consequence of excess glucose intake.

The real overlord Zenon is male
He just reincarnated to female.

There is no set canon for the Disgaea trio post-Disgaea 1
Disgaea Infinite seems to imply that all of their later cameos are based on the game's different endings and are not connected at all. One ending has Laharl eat Etna's pudding, causing her to quit and go to another Netherworld. Another has Etna trap Laharl in a strange looking bottle.

Every ending of each game is true
Its a multiverse. Each time a game comes to an end, the dimension branches, one new timeline for each potential ending, explaining the above mutually-exclusive-cameos problem. References from one cameo to another may be due to diverged timelines joining back up hypertime style.

Baal isn't just a body-hopping Dimension Lord, he's an Eldritch Abomination.
Think about it.
 * Directly stated in Makai Kingdom.

All beings have the capacity to become like Baal.
All humans/angels can become demons. All sufficiently powerful demons can become Overlords. All Overlords have the capacity to become pseudo-Lovecraftian horrors,like Zenon, or full-on Cosmic Horror things, like Baal.

Alternatively, all beings like this ARE Baal. Using their great powers, they traveled back in time and combined to become Baal. Meaning...Baal's strength is multiplied by a factor of 1000 by the second every second. Zenon was merely another potential "candidate".

Baal is a Prinny/Eyringa.
He's not hopping to a spare body, so much as returning to his original body while his Mobile Suit Ogre is being rebuilt.
 * Rather doubtable. There are (at times) shown to be leftover Prinny suits lying around, and the Eyringa are shown to be weak creatures. It's likely Baal just shoves the original soul out and moves himself in while waiting for his original body (or any duplicates thereof) to be restored... though, does his original body restore itself during that time?

Rozalin/Adell is an Overlord
Perhaps not a particularly strong overlord, but an overlord none the less. Since titles and ranks transfer from the loser to victor, Beating the (Fake) Overlord Zenon granted Overlord Status. Now as to WHO exactly gets the title is somewhat up to debate. While Rozalin dealt the killing blow, Adell did basically take him down for the most part, which may be all that is needed for the title transfer.
 * They are now BOTH the Overlord!
 * How would that work exactly? Adell's the Brawn while she's the Brain?
 * They're co-overlords. Co-verlords!

The massive demon in the Vasa Aergun attack is Mao's true form
His father is HUGE, but Mao's only the size of an average demon. But in Makai Kingdom, we saw the equally colossal Babylon transform into a humanoid form during his battle against Zetta. Thus, it's not hard to imagine that Mao's father could do the same thing. Vasa Aergun is Mao's technique of surging up to his true size and power. However, Mao is still a kid by demon standards, and considering how hard he had to work to get Vasa Aergun in the first place, it might be that he can only sustain the attack for a short time.

Or, more likely, he sticks human sized because of the lack of Colossal Overlord-sized Slaystations. Hexbox360s, however, would probably be just oversized enough though.

Vulcanus and Volcano are one and the same
Not my theory. I do not take credit for it.
 * Or perhaps related. Siblings? Child and parent?
 * Jossed. She has absolutely nothing to do with Vulcanus

Flonne has been turned back into an angel because of some calamity that happened in the heavens.
And they need all the manpower they can get. This may dovetail into the Volcano/Vulcanus theories.
 * Made less probable by Disgaea Infinite; she's back to her angel coloration because she's taking classes. Apparently taking classes lets her get her coloring back, but she doesn't get her Angel Trainee title back until she passes the test.

Lujei Piche is responsible for any DLC that involves a characters suddenly appearing in another game and having no idea how or why.
For the Lulz.

The "embarrassing" photo of Laharl from Disgaea Hour of Darkness/Afternoon of Darkness is either...

 * a picture of him on A Date with Rosie Palms.
 * This is coming from the fact that Flonne (who's about 15 in angel/demon years) says something along the lines of "I didn't know you were into that" and the way Etna (who's about 14 in demon years) is laughing. Also, Laharl grabs himself down there in the reaction image in the psp remake. Laharl is about 13 in demon years and what are most 13 year old boys starting to find out about themselves....
 * a picture of him saving puppies and/or kitties.
 * Evidence? One of the castle residents mentions something about it not being something that a demon would do. Demons in the Nippon Ichi world generally go by Dark Is Not Evil and Even Evil Has Standards.They basically just do "evil" things like pranks or be mean to each other.
 * A third popular theory is that he was cross-dressing, either forced into it or of his own will. Also consider that both of the above points could apply to this.

In the inevitable Disgaea 5 or 6, Raspberyl will join the main cast
There's a noticeable pattern between Etna's inclusion in Disgaea 2: Cursed Memories and Axel's inclusion in Disgaea 4: A Promise Unforgotten in that they were characters who were also given Modes in the Updated Rereleases of their respective games who eventually became part of the main cast in the later sequels. Though Disgaea 3 Absence of Justice has yet to have Updated Rerelease, Raspberyl has Raspberyl Mode, so there's a possibility she may be a likely candidate for another appearance outside of obligatory cameos.
 * There's a difference though. While Etna was Laharl's sidekick from the get-go, Axel was the Mid-Boss character. You could argue, of course, that it's characters who got their own story mode later on that become main characters in later entries, but Etna only got a story mode AFTER Disgaea 2 had come out.

When Asagi appears in Disgaea 4: A Promise Unforgotten, she will appear as the reincarnated singing idol from her Asagi Wars ending
She might still have the desire to become the main character but she won't automatically draw attention to it. Also, with Axel being part of main cast, she and Axel will probably be rivals.
 * Sadly jossed.
 * The jossed was jossed. Asagi has a DLC HD Sprite. And everyone who buys it is donating money to people in Japan who were harmed by the earthquake.

Asagi Kurosugi has plotted Asagi Asagiri's death ever since Makai Kingdom
The truth about Kurosugi causing Asagi's death in Prinny comes out in Asagi Wars, but it's definitely not the first time she's plotted Asagi's death or preventing Asagi from becoming the main character. Below shows how she was involved with each of Asagi's cameos:
 * Makai Kingdom: Kurosugi tipped off Zetta that he will get his freakin' bad ass kicked by the next main character and gave him the idea that getting rid of the next main character will make him the main character for all eternity. She did not expect him to not only let Asagi live but to delay Makai Wars since she knew that she'd be next line once Asagi dies.
 * Disgaea 2: Cursed Memories: Kurosugi sent the "Fight the hero of another game" bill to the Dark Assembly, knowing that Adell, the battle maniac, will not hold back against the boss and kill Asagi during the battle. In Axel Mode, Asagi attempted to get her game out of Development Hell by shooting the Makai Wars movie. Kurosugi purposely had Axel hired as a stunt double knowing that he'd screw everything up and delay the game even further.
 * Soul Nomad: Kurosugi made Asagi believe that she had gained the power to defeat Zetta by having her fans fire the Yoshitsuna's main cannon at Zetta and blowing a dimensional gate into Haephnes. Asagi's groupies are simply Kurosugi's fans trying to make her so overconfident that she ends up dead by her ego alone.
 * Disgaea 3 Absence of Justice: Kurosugi set up the entire post game by stealing from various characters or making the characters believe they had something stolen from them and spread the rumor that Baal is the legendary Overlord thief just so that she could send Asagi to go up against him, who she was hoping would kill her.
 * Disgaea Infinite: Kurosugi found out about Asagi ordering TickTock from Amazombie and confronted the deliveryman, planning on using it so that Makai Wars isn't delayed and she becomes the main characters instead of Asagi. During the scuffle, TickTock fell into a dimensional portal, which made it's way into the Overlord's Castle.
 * Zettai Hero Project: Kurosugi gave Asagi the idea to dress up as characters from various anime series and take their place, knowing that if she succeeded, she'd possibly be kill at some point during the run of the series due to her lack of powers of the character she's dressing up as. She never expected the Z.H.P. cast to run into her.

Majin/Demon Lord Hanako will a main character in a future N1 title
Given how she has been the ONLY "brand new" DLC character in the history of N1 games thus far, it seems highly unlikely she was meant to be used the one time. At the very least, she'll be back - possibly with Taro (or Etna) in tow.

Etna is a Succubus.
Look at the lyrics for her character theme and try and say otherwise.
 * Very well then. Having looked at the lyrics, I'd say she's more reminiscent of a vampire, what with the whole "sneaking to your bedside by night" and "blood drinking" bits; then again, this is Etna, the queen of Her Codename Was Mary Sue episode previews. I honestly wouldn't be surprised if she came up with those lyrics herself.

Laharl loves cats
The reason Anise the Cat Witch is in love with Laharl is because he pets her, when she is in her cat form, every time she visits the Overlords castle, because he loves cats. Using cats as Pet the Dog moment is very common in Japanese media apparently.

There will be a Makai Wars 2 game.
Not an actual Makai Wars, mind you... Instead, it will be presented as a 'sequel' to the never-released game. In a parody of Rogue Protagonist, Asagi will go Off the Rails upon discovering she won't be the main character this time, in favor of a 'new hero'. Depending on how sadistic the developers are feeling, she will either technically have the lead role even if she isn't recognized as the Main Character, serving as a Villain Protagonist out to defeat the so-called heroes that so cruelly stole her starring role from her... or she'll only have an extra Side Story mode, along the lines of the PSP games.

Adell wasn't born human, but he is now.
The distinction between humans, demons, and angels is vague and arbitrary enough that individuals can switch from one to the other with little more than a simple job/title change. Whose to say that Adell's firm belief that he's the last remaining human on Veldime, and that it's this is the indicator that he's the one that's supposed to defeat Overlord Zenon, hasn't effectively turned him human?

The Netherworld is actually a planet orbiting a red dwarf star.
It makes perfect sense: when we see the overlord castle, it is always night, and it's in the middle of a lava lake. That's because it's on a planet that is in a tight orbit around a red dwarf star, and therefore tidally locked (with the same side always facing the star). This proximity causes strong tidal effects, which cause internal frictions and explain the permanent volcanic activity (which the names of the characters match!) And the red eyes of the demons are meant to protect their retinas from the frequent flares this kind of star is known to have. This also explains the ages: a 'year' there is just a few earth days. Plus, the moon is conspicuously red (reflecting the light of a red sun) and the Netherworld can be reached with a spaceship.
 * A problem with that: A database entry in Disgaea Infinite establishes that time passes slower in the netherworld, not faster, and that ordinary demons have a life span 100 times that of a normal human as a result. In Disgaea 2: Cursed Memories, demons native to Veldime, an unrelated human world, age like humans but retain the long lifespan. This means it's a magical/inherent thing not related to orbital rotation.
 * The rest of it is quite plausible, however - an NPC in the first game points out that the netherworld is on a planet and demons would be considered aliens to the humans.

The Prism Rangers participated in the Legendary War
Which is why they don't show up in Disgaea 4.
 * Scratch that. They're back as DLC.

Master Big Star is an Incubus
He has large wings like the Succubus class and is extremely good looking, being The Dandy and all.
 * More specifically, his "Beauty is Sin" evility does empower female allies, so that could put some support towards this.

Etna is Yoko's reincarnation and Laharl is Kamina's
The resemblance is uncanny: redhair, skimpy outfit, long weapon for Etna, blue hair and shirtless for Laharl. Perhaps good humans can be reincarnated as demons (with the long lifespans and awesome powers, it sure beats becoming a Prinny...) And Flonne is Nia's.
 * Jossed by the fact that, were Etna at all connected to Yoko, she wouldn't be nearly that flat.

Baal's end
It was stated in Zettai Hero Project that Valvolga killed Baal for good. I was wondering how he managed to get rid of a mass of souls that can Body Surf, until, having read a certain fanfic, I thought: suppose Valvolga found a method of striking directly at Baal's soul collection, much like how Gig and the reaper class monsters reap souls, and just kept at it until Baal ran out of souls, making his attempt far more successful that what Laharl's would ever become.
 * Since Baal reappears as DLC in Disgaea 4, I think the implication was that Valvolga lied about "killing Baal for good" so he could get the bonus boss spot.

Conviction Gives Humans strength equal to a demons.
Confirmed in Disgaea 4 with Fuka. But let's explore anyway.
 * Saphire is determined to protect herself so that no noble warriors have to die in her name. She consolidates this Conviction into the catch phrase "First to Strike, First To win."
 * CAPTAIN GORDON DEFENDER OF EARTH and company.
 * The Unlosing Ranger is always powered by some Heroic Conviction. The Suit just let's the Human take it Up to Eleven. To where they can go one on one with Overlords.

Every Human World has its own "Constellation" of Netherworlds

 * The Human World from Disgaea 1 has Laharl's Netherworld.
 * Hades Is another, and the story involving it takes place further into the future of the original Disgaea.
 * The Various Overlords from Makai Kindgom own Netherworlds that form the constellation for 'Our' Human World. The Unlosing Ranger and other such heroes are the reason their presence isn't as felt as it is for the Disgaea Human World.

Sardines DO make you powerful
Whereas most demons have become weaker due to the lack of Fear Energy, Valvatorez, despite giving up blood, has maintained enough power to defeat anything that gets in his way. Also, while everyone was slowly being infected by the A Virus, he managed to keep himself from turning into Axel, only doing so if you lost the final battle with him.

The Earth from Disgaea 4: A Promise Unforgotten is the same Earth from Disgaea Hour of Darkness
Technology has advanced and the problems that caused the EDF to consider invading Laharl's Netherworld got worse.

In the Worst Ending of D3, Mao triggered
Remember, allows malice to be manifested into physical form. By defeating the malice,. This is the reason why And then

Laharl's mother was a fire witch
Laharl first skill is of the type fire and in Disgaea 4  first skill looks exactly the same except the skill's type is ice. Lahral's scarf is pretty fire-like too.

Disgaea 5 will be even more meta than any of the previous games.
The main character will be an angel who wants to become a demon (whose ultimate special move would involve sending the opponent through the 9 circles of hell), who will be joined by a nerd that's obsessed with min-maxxing every single character, and a literal glitch in the games programming that can pass through enemy characters (whose ultimate attack literally causes the game to suffer a BSOD). The plot would involve things such as: bad fan fiction, shipping, and mary sues. The villain will be a whiny fanboy  that thinks that the later games aren't as good as the earlier ones and  with the hope that the fans will leave the series behind and move on to better things (like remakes!). Asagi will get her own game  Also, Gwen would be a DLC.

The maturation rate of demons depends on where they primarily raised rather than their species.
However, if a demon were to be born and/or raised in a human world, then they will grow and mature at the rate of a human. Examples: Adell (age 17, pure blood) and Rutile (age 15, half nekomata). This does NOT have an effect on their life span.