Sayoko Bizen from Isutoshi's "Slut Girl" hentai manga
Okay, now you might think that it's only her looks, but she actually possess some rather interesting personality traits for a female character in a hentai. She smokes, she swears, she's bossy, argumentative and dominant to pretty much everyone. Quite simply, she's almost as bad as myself, which of course is bound to catch my fancy.
Sidenote: Isutoshi is the main reason why I tried my hand at drawing female characters. The man is a fucking genius, even if a somewhat perverted genius. I wish I could draw women the way he does. :S