I just played the game, and tbh I still don't really understand the story qwq but here's what I've put together:
WARNING! SPOILERS!!!
It appears that a person named "Piero" did something terrible to Kana. But what I don't understand, is if Piero and Shin are the same person. The messages made me assume that Piero and is the person that Shin didn't want to be friends with anymore, cause Shin said he "knows" what Piero did to her. But also, the "picture" of Shin looks like Piero's sprite? So my thought process is..
If Piero and Shin are the same person, then he was someone who "fell in love" with Kana through her blog + messaging her. It became parasocial and he murdered her (maybe in the closet seen in that one picture?)
If they arent the same person, then I have no idea TvT