Tajuujinkaku Tantei Psycho

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.