MPD Psycho is a manga written by OTSUKA Eiji and illustrated by TAJIMA Sho-u. There is a spin-off series under the name Tajuu Jinkaku Tantei Psychoco.
The main character is a detective, Kazuhiko Amamiya, suffering from Multiple Personality Disorder (a.k.a. Dissociative Identity Disorder). Starting out as a violent detective series with a twist (the twist being the variant personalities that take over Detective Amamiya at various intervals), the series evolves into an exceptionally complex and involved sci-fi conspiracy story.