I don't know if this has already been said or not but i poseted this on some other threads I came up with a theory of my own. They never did explain what happened to Xehanorts form or heartless after Riku turned back to normal towards the end of KH2. I think that when he finally spilt from Xehanorts grasp on him his heartless was reborn in Kingdom Hearts, and thats is him in the future, and the person in the red and black looks like Riku when he was wearing that weird suit from KH1 and darkness mode in CoM. But it could also be the shadow that he always had with him.