... She looks familiar, but I'm not sure who she's supposed to be. Or maybe I'm having a false positive.

EDIT: My final conclusion is that she's a new, probably one-off girl, who's hairstyle is probably just a bit similar to something that Cheer-chan had worn before in a manga chapter; this girl lacks Cheer-chan's signature sidelocks, so she can't be her. Himura makes a point of giving each of the main girls and boys specific physical features that keep them distinct from one another, and AFAIK they've never forgot to draw those features before.