Unlocking characters for Free/Combination/Survival Mode:
You have to do the following in story mode:
Asakura Saya – Find all the evidence.
Doi Junpei – Finish 1st stage in story mode.
Nakatugawa Kazuya – Examine all evidence.
Ogata Shunsaku – Finish 5th stage in story mode.
Sima Ryuji – Finish 2nd stage in story mode.
Honjo Yuichiro – Collect all the letters from all the people you can save.