Comment by Morrigan on Heiwa

. In Chapter 3 (“Liar”), Yamabuki explicitly compares Rikka to Apollo, and Rikka echoes Apollo’s on-air phrasing/apology about being off-key—an authorial nudge that she and Apollo share a voice and habits. 

Rikka is repeatedly tied to heartbeat and moon/celestial imagery (chapters “Celestial” #30). This tracks neatly with the “Apollo” (celestial) motif.  There were more hints and she even referenced Apollo during summer break. 

Apollo’s show restarts only after Yamabuki joins the broadcasting club—i.e., when Rikka gains proximity to him again. Of all the candidates, Rikka’s role best explains this trigger without requiring extra contrivances (she was the first to make contact with him for seemingly no reason after the school suddenly became co-ed and girls were not sure about interacting with guys). 

If Rikka turns out to be Apollo, she will simply have everyone say "oh yeah, that makes sense". The other one, which is a sideliner and not main, is Shinobu only on account we really don't know much