I know I've found this in the past, but I seem to have lost my references. I think I originally read this in early 2000, in English, online. At some point, that website went down, I found some cached copies online, and eventually found the original author (I think through associating his name with an XBox Live account) and received a dump of files from him that included all of the parts, and a few of the comic images.
The basic concept is that a young man in college wakes up one morning to a succubus who tells him something along the lines of "Good morning, I'm going to kill you now and eat your soul", and he manages to convince her to give him one last roll in the hay before killing him, whereupon she (who I think it was implied had very little actual carnal experience) decided she could get a bit of his soul at a time by having sex with him, and that she could spread out eating his soul over several months. She puts her brand on him (which shows up as a super-deformed image of her flashing a peace-sign as an embarrassing tattoo, I think on his hip) and when he attempts to escape, he finds out she's put a magical leash on him that manifests if he gets more than a certain distance from her. This results in him having to take her with him to class.
One early character introduced was one of his professors, a man with a pronounced German accent and an obsession with people being on-time. Because the demon girl makes him late that day, he expects to get in trouble, but upon seeing the demon girl (who uses some sort of magic to appear more normal), the teacher softens, which the protagonist finds very odd. Other characters I remember were several girls after the main character (the plotline moving to sort of a harem setup) included a juvenile delinquent (who I think is a prominent Judo user) who sees him as a good catch because he defeated her in combat (I think, accidentally), a cousin of his who died before the events of the story and is now stalking him as a ghost, and an angel who used to go to school with the girl (with, I think, there being a bit of a plot twist in a flashback where they started off on opposite sides, with the demon as an angel and the angel as a demon, but wound up switching places somehow).
A later plot development is that the demon girl is slowly sickening and dying, as it turns out she can't get the soul sustenance from sex that she thought she could, which leads to the main character planning to sacrifice himself for her survival, and the German teacher eventually revealing himself to be aware of the existence of demons and he's either a fallen angel or a former Nazi mystical researcher, who plans on trapping the demon girl and somehow extracting her power. I seem to remember that there was something involving a pocket watch he carried having mystical properties, maybe freezing her in place.
I think it had a happy ending, with the boy winding up with the demon girl, but I don't remember how that happened.
The demon girl had the stereotypical "succubus" setup with bat wings, horns, and a tail with a spade tip. The angels had halos and wings. I don't remember the sex scenes being all that steamy, although I think there were some descriptions of her nude body. The webcomic was a bit stiff and amateurish, and was definitely done in an anime style.
