Nightwalker: The Midnight Detective (真夜中の探偵, Naito Uōkā Mayonaka no Tantei?) is an anime created by Ayana Itsuki that combines elements of the horror, detective story and vampire genres. The anime is based on a PC game titled Mayonaka no Tantei Night Walker (真夜中の探偵 Night Walker, Mayonaka no Tantei Night Walker?), developed by TomBoy. 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. Music is written by Akifumi Tada.

Source: Wikipedia