She a replica, like Riku Replica, made out of Sora's leaked memories. She takes shape as a girl who looks like Namine and Kairi because she takes shape of Sora's strongest memory, Kairi. Xemnas created her with the intention that someday, she would fight Roxas and absorb him. She would be the new Sora.. And Sora would be stuck in his pod forever. In the end, she started turning into Sora..literally. She gets a little crazy and fights Roxas willingly.. Roxas wins and absorbs her. That is also how he gained dual weilding, because he uses Xion's keyblade now.