You have to do the following in Story mode.

Unlockable	How to Unlock
Asakura Saya	Find all the evidence.
Doi Junpei	Finish 1st stage in story mode.
Honjo Yuichiro	Collect all the letters from all the people you can save.
Nakatugawa Kazuya	Examine all evidence.
Ogata Shunsaku	Finish 5th stage in story mode.
Sima Ryuji	Finish 2nd stage in story mode.