Nightwalker

In the world of Nightwalker, attacks by the nightbreed minor demons that possess humans and feed on their souls, are relatively common. As a group, nightbreeds (often referred to by the abbreviation 'breed') are usually self-serving and of low intelligence, and they drive a human host to commit crimes. Because these breed-inspired crimes are often difficult to identify, a special agency known as the N.O.S. has been organized to investigate and solve breed-related cases.
Shido, the main character of the series, is a vampire who decided to defend humans against his demonic brethren as atonement for the sins he committed early in his vampiric life. He knows little of his life prior to becoming a vampire, and even his name is a pseudonym he took when he arrived in Japan. Upon settling down in Japan, Shido established a one-man (plus one-secretary) private investigator office, which he uses as a means to hunt breeds while keeping his true nature secret. Shido even keeps his identity secret from Riho, a teenage girl he decided to employ as a secretary after her parents were killed in a breed attack.
All is not business as usual at Shido's office, however, as Shido is soon forced to reveal to Riho that he is the same sort of creature that killed her parents. Furthermore, Shido's powerful and obsessive sire, Cain, has plans in motion to reclaim Shido, as well as to initiate the 'Golden Dawn', a secret with far-reaching implications for all creatures of the night.