Vampire Dormitory 7 (manga)

Mito, who has no family to rely on, lives on the streets, disguised as a boy. Ruka, an otaku vampire who's only interested in 2-D girls, saves her from a perilous situation and makes her an offer: become his subservient thrall from which he can feed whenever he wants, and she can live with him-in the boys' dorm. Because her very existence depends on her secret not being found out, every day is a new dangersay nothing of that vampire! Meanwhile, Ruka, not knowing Mito's a girl, dotes on her night and day in an attempt to ripen her "disgusting male blood," but when real feelings develop...

this dangerous romance between a crossdressing girl and an obsessive vampire begins!