Huh, I honestly thought Mio is the magical girl you meet in the story. As she appeared right after her, the memories seem fake to me jumping around with the main character even questioning it. Other character's don't remember her until they meet and there is no physical evidence she actually was there before, but she isn't acting like that is weird but instead is directing attention away from that. One time the main character is even about to question things it glitches and suddenly she no longer is questioning it as had a memory to fill it in. It clearly seems like she is a character who has that ability, and is using it to insert herself close to the main character. The magical girl just met seems like the obvious choice, as perfect way for her to get close to Charity and be a bad influence. The little we have seen so far of her seems like that is the case but it's a bit early to say there.
If this is the case it feels like it's being done pretty well so far, but also a bit early to say from what I can tell from available content. (actually reminds me a bit of a horror story heard a while ago Fleshgait I believe was the name)