On 30th of May 358/2 Days was released in Japan and it answerd the question about who Xion is
*Spoilers start here*
She is not actually a Nobody, but a doll created by Xemnas from Sora's leaked memories. Her face was based off of Sora's strongest memory, Kairi, but as Naminé pieced together Sora's memories, she began to physically look like Sora.
The plan was for Xion to eventually become a complete Sora replica, due to the fact that Xemnas was unsure if he could rely on the actual Sora to gather him enough hearts for Kingdom Hearts. Eventually...