La Pucelle/WMG

After the game, Prier became a Demon Overlord to prevent another Maiden of Light/Dark Prince war.
Yes, she helped Croix the Dark Prince fight his Enemy Within, but it's doubtful the Fallen Angel Calamity would give up that easily. Prier finds out that Calamity is after Croix, and so she takes steps to become the Maiden Of Light... and then opens a portal to Hell and kills enough demons that she becomes the Demon Overlord for her world. By substituting the Demon Overlord system for the Maiden/Dark Prince one, she challenges Calamity and takes over for evil. To maintain the dark energy needed to continue being a Demon Overlord without harassing her birth world, she challenges Alternate Netherworlds to fights, and main characters end up facing her in the Disgaea games. However, she has to cultivate a reputation of being a Blood Knight.
 * Supporting this is Demon Overlord Prier's title as "The Fallen Maiden." Her special attack is also a "dark" version of the Maiden of Light's attack.
 * The Dark Prince and the Maiden of Light go together. If Prier becomes the Maiden, then Croix would have to turn back into the Prince, which is exactly what Prier is trying to prevent. As said above, since her titles keep referring her as a Maiden, she may have joined the side of "evil" to stop that war. By becoming a powerful Overlord, even if the Dark Prince did come back, she could probably handle it without a repeat of the last time.
 * The only problem with that theory is that Disgaea 2 insinuates that Priere destroyed Paprica after becoming an Overlord.
 * Not necessarily the case. It could be that, after Priere did away with the Dark Prince/Maiden of Light system, that Paprika wasn't too pleased. Everyone had been doing this for so long, how else would they go about things? Logically, most people would want things to go back to the way they were (From what the people have known, demons= bad and scary, remember? And since Priere is one...). So, I think that they went on to try and do that, only to have Priere descend and blow shit up to stop them from re-instituting it. Of course, everyone dying or not is completely optional to each tropers' personal opinion of Demon Overlord Priere.
 * It's absolutely stunning that this was all created before the Ragnarok came out; because it's ALL true. I Knew It!.

Even without the bonus content, Prier is doomed to be a Demon Overlord... or worse.
The N1 universe has a simple rule, you defeat it, you become it. This is why the Maiden has to die once she uses up all her powers. Since she exists only to defeat the Dark Prince, if she didn't die, she would accidentally inherit his title and eventually turn to darkness herself. This was done to prevent such a thing. However, Alouette and Prier found a loophole, and technically Prier is the one who defeated the Dark Prince, so she will eventually lose her soul to the darkness and become "Overlord Prier".
 * That her Alternate Netherworld in Disgaea 2 is called "Ruins of St. Paprika" may imply that she destroyed it.
 * Not sure. I've always assumed the 'Ruins of St. Paprika' were in reference to the sewers/Montblanc castle (LP Chapter 1), mostly because it has a sewer feel to it.

"Sea of Blood" Prier was faking it all along.
Turning into a demon isn't enough to explain such a radical change of personality if you take Disgaea 2 and 3 transformations as examples. Prier had some minor changes after being crowned Overlord, but for the most part, she's faking the whole thing in order to sound tough. She was already doing the same thing with her own brother, going all Stepford Smiler when the subject of her dead parents came up. While it started with her brother, she pretty much doesn't want to look weak in front of others. She's doing the exact same thing in the Netherworld, especially since being the strongest is the only thing that's important. So she's just going around, trying to act like the demons in her Netherworld as a facade. The only problem is that it's pretty much obvious that the demons in La Pucelle's Netherworld act very different from the demons in Disgaea. The fact that Prier goes back to her normal personality in Raspberyl's Mode in Disgaea 3: Absence of Justice seems to hint that she's still herself.
 * It should be noted that in the PSP Video Game Remake of Disgaea 2: Cursed Memories, the whole "Sea of Blood" thing was removed.
 * Sea of Blood Prier seems to be the extension of Ragnarok's bad ending.

Noir is... Asagi
Forget being punished for his sins by turning into a Prinny, that's not enough. Instead, his human soul got reincarnated into Asagi and forced to endure game after game of slowly going insane due to crazy demon antics and getting screwed over and over. That's the ultimate punishment for trying to destroy the Gods, never being a main character in a series where being the main means everything!

Two things: A human lifespan is nothing compared to a demon's, and as Makai Kingdom showed us...
...humans can be reincarnated as demons after they die. So when a bunch of demons called Priere their Overlord, she got annoyed, forgot about it, and went on with her normal human life until she passed away. Upon where she was reincarnated as a demon like Seedle and Salome, and the demons that pronounced her the Overlord were waiting for her. So in the end she accepted because it's not like she had anything better to do. Eventually she went back to her own world, centuries later where her civilization had collapsed, and took it over as a benevolent ruler dedicated to creating human/demon harmony, so that another person like Noir won't have to show up and simultaneously ensuring a balance of light and dark. Portreene being an "incarnation of light" pretty much just exists at this point, and Calamity ether vanished or was beaten the holy heck out of. Her OOC-ness in the Disgaea games were the result of a bet: "Fine, am I acting like a demon enough now?"

As for Eclair's downloadable in PSP Disgaea 2 and the Disgaea levels in PSP La Pucelle, respectively...Baal is probably mucking about with the timestream now.
 * Nah, the last one is just Lujei being bored like hell again.

There is both "Crazy Sea of Blood" Demon Overlord Prier, and "Maiden of Darkness" Demon Overlord Prier.
Disgaea Infinite has Multiple Endings leading to events of various games; heavily implying that each event is an Alternate Universe shard. The same goes for Demon Overlord Prier. There's one where she went insane from the power of Darkness, and one where she did not.
 * Way too easy on the writers, in my opinion, but it fits what we've seen of "canon" so far.
 * While we're at it, there's probably still a "Human Prier" somewhere too. Let's just hope the three don't meet up.
 * They already have. This is why "Good" Demon Overlord Prier is at the Ruins of Paprica. She killed Sea of Blood Demon Overlord Prier in her Alternate Dimension; and then the protagonists show up.. (Which is why we haven't seen Sea of Blood Prier since PlayStation 2 Disgaea 2: Cursed Memories.)

Hero of Darkness Croix inspired the legend of the Dark Hero in Disgaea 2: Cursed Memories
The Demon culture got a lot of the story wrong, and made up their own parts.
 * Thank you, now I have the mental image of Croix ding-dong ditching Alouette...

Poitreene and Calamity are the same entity and the world of La Pucelle isn't a human world
Light and darkness are just two sides of the same coin. Poitreene is the personification of pure light and Calamity is simply her Super-Powered Evil Side that started to form in her heart after seeing all the evil humans could do. The world La Pucelle takes place in isn't an actual world, it's Poitreene's heart. The battle between the Maiden of Light and the Dark Prince is simply the two personas fighting to see who gets control of the heart, the other humans being "heart residents".

There is still a sliver of hope for Priere and Croix.
This relates to Disgaea 3, where Priere and Marona meet by post-game content. Please recall that Marona can recall the dead, [I]even if their soul has been destroyed.[/I] Whilst her heart has been lost and she wouldn't do it at first....afterwards, there's a chance. Would require Priere to open up about it though, and that's unlikely at best. There's also the risk that his soul would be all mixed up with Noir, given they both were obliterated in the same area. Would make for a heck of a boss fight. Anyway, the fact that they have actually met makes this possible, though not quite probable.

The Dark Prince is Fear the Great.
The similarities are quite obvious. While Flonne said that Fear the Great had never been stopped before, she did say she read it in a report, so the failures could have erased. Which leads to...

The Maiden of Light is a virus possibly created by Calamity

 * Which means that somewhere down the line...