Well, I don't know the whole story, but it is Roxas trying to find out why he can use the keyblade, and soon he is joined by Xion who can also use a keyblade. As Sora is regaining his memories it is affecting Roxas, but it is also affecting Xion. She is infact a copy made of Sora's memories just incase Sora can't be used by XIII. Sora, for some resion can't awaken becase of this. After some time, Roxas leaves and Xion is reprogramed to absorb Roxas and becoming the "real" Sora. The two fight and Roxas wins. I think Roxas absorbs Xion, after she told him to stop XIII. One the way back to The World That Never Was he fights Riku, and you know the rest.