Urotsukidōji: Legend of the Overfiend

Legends say humanity isn't Earth's sole dominant civilization. Two other races share this world: the Makai, a demon lineage, and the Jujinkai, a half-man, half-beast order. Every 3,000 years a supreme being known as the Choujin, or the Overfiend, rises to restore balance among the three realms of Earth. In present-day Japan, after three centuries of relentless searching, a Jujinkai named Amano Jyaku uncovers the Choujin's presence within a high school slacker named Tatsuo Nagumo. Now Amano, alongside his sister Megumi and their devoted ally Kuroko, must shield Nagumo and his new girlfriend Akemi Ito from the Makai, who believe Nagumo is not the Choujin but an evil entity bent on destroying all living beings on Earth.