Maaka Karin comes from a family of vampires living in Japan, but she is not a normal vampire. Vampires are normally supposed to drink the blood of their victims…but Karin suffers from a condition that causes her body to produce excessive amounts of blood. About once a month, she has to get rid of the excess blood by giving it to her “victims.” But she somehow manages…she goes to school like everyone else her age.
Well, things are about to get more complicated. When a new student, Usui Kenta transfers into Karin’s class, she starts producing unusually huge amounts of blood, almost as if in reaction to him. Worse, he happens upon Karin’s…true nature…
Descriptions taken from AniDB.net