I'm kind of dissapointed that there was background music but no sound effects to for any actions scenes. Makes it less dramatic to me. It's still looks awesome though.
I think this scene is taking place during the "keyblade war", which means that this is all occuring in the past. Which means none of the "knights" can be Roxas, since he is really Sora's Nobody and would not exsist before him. We are dealing with new characters here.
However the similarities in appearances to Sora, Riku, and Kari, leads me to believe that they may be their ancestors. The original bearers of the "keyblades". Roxas also looks like Sora btw.