Let's make her a teacher who died in the mass suicide. The reason she's different from the other spirits is because unlike the other spirits who committed suicide, she was murdered. The reason behind the murder could be

1) She did not agree with the idea of a mass suicide and tried to stop it. So the others murdered her.

2) The mastermind was unable to manipulate her as he/she was manipulating the others. Red Lady was becoming an obstacle to his/her plan so the mastermind her out.

3) She was teacher in that school but not a part of the mass suicide. Was investigating the reason as to why her students and fellow teachers committed suicide. Was getting close to the answer, so she was killed by the mastermind and he body was disposed, which is why no one knew of her death.

Has any details of the mastermind been decided, seeing that it's most likely going to be the final boss?

There's something that's left me confused. Shiina Ai said that Red sincerely was helping the player and wanted to be put at rest, but then tries to kill the player as she feels he's useless and at the Hour of attack him at the Ox hour, when she's vulnerable? Why would she want to kill the player at the time she's vulnerable and at other times? Is it because she's vulnerable and thus becomes like the other spirits who follow the rule of kill intruders? Could you explain the details? Or does it have to be discussed?

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