Karin: Chibi Vampire

In the 21st century, a family of vampires settled down on the grounds of Japan and lived a modest life whilst sucking on the blood of humans discreetly. However, the eldest daughter of the family, Karin Maaka, has an embarassing secret which cannot be revealed to anyone. Even though she is a vampire, she does not lack blood like her family members. Instead, she overproduces blood to the extent that she transfuses the excess into her victims, hence increasing their blood levels. Despite her troubling body characteristic, she attends school like any other high school student, that is until a new transfer student, Kenta Usui, enters her life.

Synopsis from Anime News Network