Namine being Kairi's nobody and Roxas being Sora's, I think we could all just chunk the idea of Xion being a nobody of either or both of them. I'm going to have to go with Xion being made up of some of Sora's memories. She looks like Kairi. She can use the KK like Sora. I just think she was made of memories that were important to Sora or something? In CoM the memories Sora lost had to go somewhere. Maybe the really important ones that stayed in his heart made Xion at one point in time? Not sure when. But that was my brain train. Or me rambling out whatever comes to mind if my terms confuse anyone.
((Though now that I think of it, I think someone else has said something similar.))