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...
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.
Betty and Veronica: Anastasia is Veronica, Hina is Betty. Izumo eventually chooses Veronica.
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: 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.
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.
Fire and Brimstone Hell
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.
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.
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.