I’m curious about MC and Tasogare’s past. I’m sure they met before at some point. But it feels like the MC either lost his memories or had them altered, just like when he met the other girls
My guess is that she's a time traveler. In the second part of the first chapter, she said "getting in was the easiest its ever been. Only this time... what's different?" She was talking like she had done it multiple times before.So it was definitely the song that brought her here. That magic circle has been mentioned to many times for it to not be significant so I'm gonna assume it has something to do with the magic girl.
I'm gonna assume that Arataka is a special case in the sense that her reason for getting involved here is more ordinary only because she's getting slightly special treatment already when it comes to his relationship with her.