Nah, I don't think she's a monster. Since this chapter reveal she's been little kid before and has a family, she's very likely a human. Probably an exorcist of some sort, a monster hunter. Or the opposite. But anyhow her appearance nicely concludes Shiori's train of thought that someone's probably been looking after Hinako.
Should have just said to Miko "yeah, I had just gotten home and that crazy bitch from school dragged me out to the festival - it was annoying as fuck" instead of le-typical-useless-lesbian-keeping-dumb-secrets.
Place your bets now! An other monster or exorcist? I'm going for exorcist. Like someone in the comments said, seems the most logical one seeing as the main MC hasn't been eaten yet.