MPD Psycho - Vol. 3 Ch. 18 - Ghost

[MangaDex]

 

[zawarudo]

That's an important hint. My guess is that aside from Shinji, Yosuke, Kotone, Minako, Kazuhiko and Kiyoshi, there was actually an as-yet unidentified seventh person in the Amamiya Clinic, who is the MC's real identity.

Shinji is The Killer.
Yosuke is The Cop.
Kotone is The Girl.
Minako is The Nurse.
Kazuhiko is The Nerd.
Kiyoshi is The Fool.