I have an obvious but probably dumb thought about the flames. Had there been a cross in the flames like a normal heartless symbol, they kind of would have gone through the Dual Wielding Unknown. I'm just saying. DJzen makes a good point about the flames though. (I think Riku is the blindfolded unknown too)

My thought on the blonde kid is also that he is another keyblade wielder. But more specifically, I think he is a previous keyblade wielder. According to Leon (Squall), the heartless have been around for at least nine years. I don't know why the keyblade would have waited for that long before choosing a keyblade wielder. I don't know why, but I just have a feeling that all of those scenes in the KHII trailer of the blonde kid are scenes from the past. Somewhere along the line, maybe during that fight with the red haired unknown, he lost his heart, which made him an unknown. And possibly he lost that fight at the same instant that the keyblade chose Sora. I don't know. I know my thoughts leave a lot to question, but I just have this feeling that this is the case.