What. According to this picture, the pawn on c5 is from a7... Which means that two pieces were taken, with the only viable pieces being either rook (since they're offscreen) or a queen... ...I declare this to be a blasphemous chess match D:
Is... there a joke I'm missing or something? I'm not clear on what the KoReimu has to do with anything. Is it one of the players? A makeshift piece? Just sitting there being angry as fuck that a chess game happens to be going on around it?
Or maybe the idea of this chess game is to literally "kill the black king" in this case the reimu. So when you check mate, you stomp it with the pieces or something.
She's clearly in check and quite angry about it, so that maybe a hint that something not nice may happen to it if check mate is achieved.