The Ultimate Vampire Resource and Directory

The main character is Shido, a vampire who has promised to protect humans. A private investigator by day and a demon hunter by night, Shido solves murder cases caused by nightbreeds (low-class monsters) with his friends Riho Yamizaki, Yayoi Matsunaga and Guni.

In the world of Nightwalker, attacks by the night breed, minor demons that possess humans and feed on their souls, are relatively common. As a group, night breeds (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.