Me, Myself, and I
multiple personality detective

Kobayashi Yousuke
This is the original personality of the detective when he appears at the beginning of the series. He was a police detective, though not a particularly good one. He has a weak stomach at crime scenes. While he was working on a serial killer case, the killer tracked down his girlfriend, kidnapped her, cut off her arms and legs and put her on an IV. Then he delivered her, still alive in a cooler, to Yousuke's office. Yousuke disappears after the killer's death.

Amamiya Kazuhiko
The main personality of the detective in the manga is this detached criminal profiler. He's very good at what he does, and the police often come to him in prison for help. This is what gave Machi, a former police officer, the idea to quit and start a Criminal Research Center with him.

Nishizono Shinji
By far the most disturbing of his personalities is this killer, who cruelly murdered the psychopath that killed Yousuke's girlfriend. (And other people too, but not until later on.)

Murata Kiyoshi
A stuttering druggie, in the past he used Kobayashi's knowledge of planned busts to warn his friends. He also warns Ueno Suguru when the police plan to search his apartment for the missing women.

Amamiya Mina
A blind young woman, age 16. She appears for only a short time.

Spoilers