Vampire Dormitory 

Mito, who has no family to rely on, lives on the streets, disguised as a boy. Ruka, a vampire, 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 danger—to say 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! (Source: Kodansha USA)