Yeah, I don't think any of us have any idea who she is. Maybe there is some kind of connection with her to what happened before, but she would've been just a kid when everything happened, since she looks maybe around 16 or so now... unless she's older than she looks.

All I know is that this Hibiki she could be aligned with now is dangerous. Maybe it's possible they somehow brainwashed this girl into believing the same as them.