Out of all of the hearts Sora has inside him, only one would actually think that they really are him. It can't possibly be Ven, his heart is 'sleeping'. And it can't be Roxas or Namine, because near the end of kh2 they become whole with Sora and Kairi. So we can assume must just be sleeping like Ven's, or otherwise out of reach of Sora's mind. Besides, it makes no sense for either of them to think they're Sora.
Which leaves the person who was made to be Sora. Xion.
If at the end of Days she was able to look like him after just obtaining a bit of his power through Roxas, then think of what being right there in Sora's heart would do to her! This is what she had been programed to be. In the sleeping worlds where Sora and Riku are asleep themselves, it is very possible that Xion's mind took over instead, where it couldn't in the waking worlds. And(if my 3DS will post the screenshot) if you look at this, Xion is seen talking to Sora, who looks like he's wearing his kh2 outfit. That might just be nothing, but you never know.
There's absolutely every reason to believe it's Xion. So, can you see it too?