My Succubus Girlfriend

Izumo: "People have been scared of me ever since I was a child. I'm silent, and my eyes look scary. Because of that, rumors have been spread. In reality, I'm just socially awkward." (Tears start to form). "Damn, at this rate I'll never get fun friends or a cute girlfriend..."
Anastasia: That boy is perfect for my first target as a succubus!"

- "And that is how my girlfriend and I first met..."

Meet Houryuuji Izumo, your typical shy, introverted high school student. He's smart, handsome, and seemingly with no guile at all, but lonely and distant because he's, as he himself puts it, "socially awkward", unable to make friends or speak to girls. Now, enter Anastasia von Drakenvell, a succubus spawned from Hell itself (very recently, it seems), on her first mission to find a vulnerable human, seduce him and enslave his soul. A virgin like Izumo seems a perfect target.

Unfortunately for the novice succubus, Izumo is a little too pure, and seemingly impervious to her attempts to seduce him. Fearful of facing an "eternity of torture" should she return as a failure, Anastasia refuses to leave, and Izumo is far too nice to try to force her. Soon, Hilarity Ensues. Big time.

My Succubus Girlfriend is a Comedy/Romance Web Comic by Japanese creator Merryweather. Originally intended to last 45 strips, it quickly gained over 300,000 subscribers on Webtoon, and a fundraiser was started on Patreon that eventually led to a second season. It can also be viewed here on YouTube.

Still in progress, and updated occasionally.

Tropes used in My Succubus Girlfriend include:

Academy of Evil: Anastasia claims she attended a school for succubi in Hell; she also claims human schools aren't all-that different.
Affably Evil: Lucifer himself seems to be this, his first confrontation with Izumo and Anastasia being to thank them for his newfound freedom. He leaves out the part about a demonic assassin coming to kill them...
Alpha Bitch: The "mean girls" are a small group who think Anastasia is an easy target. She isn't, but one of them has a boyfriend who will not let it go unchallenged.
Bad Boss: Anastasia at first believes she'll be punished with "an eternity of torture" if she returned empty handed. Eventually, this seems to be a subversion, as Lucifer is actually willing to give her a second chance. However, it turns out this is just Lucifer's stand-in; the real one does punish minions this way.
Badass Pacifist: Izumo becomes this about halfway through season one, after the gym teacher insists he enroll in the Kendo class. He learns pretty fast, and while still not the violent type, picking a fight with him becomes unwise.
Badass in a Nice Suit: Lucifer wears a black leisure suit; archangels like Samael where spotless white suits.
Betty and Veronica: Anastasia is Veronica, Hina is Betty. Izumo eventually chooses Veronica.
Bully Hunter: Anastasia eventually becomes one inadvertently, using her powers to "convince" Izumo to stand up to them and increase his popularity.
Cain and Abel: Lucifer is Cain to Samael's Abel, Lucifer referring to God himself as "dad". How literal this means has not been explained; Samael certainly doesn't claim to be the Messiah.
Celibate Hero: Izumo is this at first. Even after confessing his love to Anastasia he rarely seems aroused.
Cooking Duel: When Hina first confronts Anastasia, she comes looking for a fight and Anastasia is happy to oblige; this nearly gets all three of them arrested, and Izumo insists they fight in some way that "doesn't inconvenience" him. They do so with video games.
Cute Monster Girl: True for both male and female demons. Succubi like Anastasia have small horns and tails, but seem human (and well-endowed) otherwise. Lucifer himself is a Bishonen, while his stand-in borders on Hot God territory, being hunky and muscular.
Dark Is Not Evil: Anastasia goes from dark to a A Lighter Shade of Grey over the course of the story.
Darker and Edgier: The first season was mostly comedic, focusing on Anastasia trying to woo Izumo despite being a Fish Out of Water. The second, however, has taken a much darker turn in a plot where Izumo's actions have inadvertently sparked a demonic invasion.
Defeat by Modesty: Anastasia does this to the "mean girls", using her magic to destroy their clothes, leaving them humiliated in front of the whole school.
Devil in Plain Sight: Several literal examples. None of the demons - Anastasia included - have any trouble telling mortals who they are or where they came from. The closest one of them comes to lying about their origins is when Malphas first appears:

Teacher: Where were you from again?
Malphas: Hell.
Teacher: Hell? Is that in Russia?
Malphas (shrugs) Sure, why not?

Do You Want to Copulate?: Anastasia's first attempt to seduce Izumo is anything but subtle, rising out of a pentagram that appears in his room, wearing dominatrix-style clothes, starting that she's going to steal his soul, and demanding sex. It fails to impress or persuade him at all, even though he claims - to himself - that it's terrifying.
God's Hands Are Tied: God seems unwilling or unable to do anything to halt Lucifer's plans, leaving it to lesser angels, who tell Izumo the obvious: "God moves in mysterious ways."
Good Costume Switch: Zigzagged. Anastasia's clothes at first seem standard "demon fashion", with black leather, stiletto heels on knee-high boots, metal choker, whole Evil Is Sexy getup. She ditches it quickly in favor of more practical clothes, but in Season 2, it's shown she can conjure up the old look if she really needs it.
Good Is Not Nice: The residents of Heaven are arrogant, rude, and full of hubris. Ironically, Lucifer comes across as far more polite than they are.
Fire and Brimstone Hell
Fiery Redhead: Anastasia is not very subtle, to say the least.
First Snow: Naturally, it does not snow in Hell, and Anastasia is shocked to see it happen when winter comes.
Fish Out of Water: Anastasia is, at first, completely out of place and ignorant of human customs and norms. In an early chapter, she barges into Izumo's classroom and loudly announces her desire to have sex with him. Naturally, a lot of students and faculty get the wrong idea.
Hell-Bent for Leather: Seems to be a standard uniform for a succubus; Anastasia wears it at first, and Valaria does so all the time.
Hell on Earth: Lucifer's eventual goal.
Horny Devils: Anastasia, of course, but also Valeria, a more experienced succubus who comes to try to set Anastasia straight. Malphas, however, despite being female, is not this type of demon. Anastasia doesn't even recognize her.
I Want Grandkids: Izumo's parents take this to ridiculous levels, not caring in the least that this strange girl they just met is living in their house, sharing their son's bed, and wearing revealing pajamas and lingerie. They're just overjoyed he finally found a prospective wife, even though he doesn't feel that way. Naturally, this does little to help him get rid of her.
Love Triangle: Anastasia's attempts to seduce Izumo cause another girl - Hina - to admit her feelings while trying to save him from what she sees as a vile seducer; Izumo has to break the two up more than once.
Magical Girlfriend: The whole point of the plot.
A Man Is Not a Virgin: Defied. Izumo doesn't even try to hide it.
Minion with an F In Evil: Anastasia makes what is clearly a succubi's version of a rookie mistake by getting two attached to her intended prey. While one attempt is made to set her straight, this only cements her eventual Heel Face Turn.
Not Distracted by the Sexy: Izumo is un-aroused and unimpressed by any and all attempts to seduce him. Anastasia goes so far as to share his bed (uninvited) and he doesn't react in the least. He can shrug off Valeria's emotion-overloading powers effortlessly.
Nice Job Breaking It, Hero: The first season ends with Izumo storming through the Gates of Hell, pointing a crossbow at Lucifer and demanding Anastasia's release; clearly he came expecting a fight, but he doesn't get one, Lucifer seems to give up and tell him he can have her! It's not until the second season where it's discovered Izumo's brave-but-rash act (which Heaven believed was impossible) violated an ancient contract, and as a result, Lucifer is now free to unleash evil upon the mortal world unopposed. Oops.
The Power of Love: Succubi like Anastasia have this as a literal superpower. They can fill a victim's head with erotic and sensual images, overwhelming them with emotion. It's strongly hinted that Anastasia uses this to convince the principal to let her attend Izumo's school as a student, and later uses it on a student making unwanted advances on girls. He later becomes a priest as a result.
Virgin Tension: The reason Anastasia targets Izumo in the first place; it seems virgin souls are valued in Hell.