>>543707234
I agree Rinha seems most likely, mostly due to the "actually an idol" part.
But I'm not gonna rule anyone else out either, they'd just need more story steps to get there.

Yuu's not an idol, but she could get bullied into changing tracks.
Kanae's not a student, but she could enroll just to make sure GKP doesn't get any funny ideas during school hours.
The other gokugetsus are idols, but they've got the voice thing. But I choose to ignore that because I wanna roll for Nadeshiko at some point.

I don't think they'll all happen. But if the team wants to get them in, there's always a path.