From Wikipedia:
On Itogami Island, a man-made island south of Japan which has developed into a monster and demon sanctuary, Kojo Akatsuki is suspected of being the Fourth Progenitor, a powerful vampire that could potentially disrupt the balance of power among the world's three ruling progenitors.
Kojo was an ordinary high schooler before becoming a vampire and is reluctant to use the powers which he barely controls. Yukina Himeragi is a neophyte Sword Shaman whose first mission is to 'observe' him.
Before long, Kojo is the centre of attention of many who are concerned about the risks he represents. Kojo and Yukina befriend various students and others, some of whom are attracted to Kojo, and whose situations drive his libido, which provides the source of much of his power.
