Vampire Dormitory 1 (manga)

Mito has no family to rely on. She lives on the streets, disguised as a boy. Ruka is an otaku vampire interested only in 2D girls. After Ruka saves Mito from a perilous situation, he makes her an offer: “Become my subservient thrall and let me drink your blood whenever I wish! In return, you can live with me—in the boys’ dorm!” But to stay with him, Mito must hide the fact that she’s a girl. Every day is a new danger—to say nothing of that bloodthirsty vampire!