Sooooooo. That’s UU’s symbol. And he’s got grey skin, and since this is Homestuck I’m guessing that’s not just artistic license or book covers being weirdly colored. I don’t see any horns though. 

The chess match obviously refers to the Skaia battles, so I assume in the future we’ll see the king (the white king is the one we like, right?) die and the fight somehow continue.