He's probably a boy, since he's seen in flashbacks wearing a male school uniform and a more boyish haircut, but in current times he presents as a girl, hanging out with an otherwise all-female group and wearing girl's clothes. But he also uses the masculine "boku" when referring to himself. And has a "secret" he's been worried about telling to the others for the past few events centered around his band, even getting a song with a 3D MV about his dilemma that has some extremely subtle colour-based hints about what the secret might be.

Eh, lots of anime does that trap androgynous male/female thing, but usually they're subtle about it. This seems a bit borderline in your face.