Only thing I can think of is that Chiaki's corpse got possessed so now she has her original personality and the evil spirit's personality and powers...?
It's fairly obvious that Chiaki died tragically/violently, and that Ryouhei has some repressed childhood memories of the event or the aftermath.
I think her ghost needs him to assist her in some way - maybe just transport her but likely more - in her pursuit of either vengeance or redemption. And glasses guy has something to do with her death.
An interesting bit that hit me when I started writing this - Yasu is his childhood friend, so it's possible that she knew Chiaki then too, and it's almost certain that she knows more about whatever happened.
I already know of that thing since a fair share of time, so you linking to that went just according to keikaku, although truth be told I was thinking more of this one:
Ookay, this thing is legit a wraith. Someone should call exorcist, Arataka Reigen, little girl Hozuki, that Asakura guy, whoever is fine. MC is in danger of dying a stupid death.