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One of Xion's clones who are still in the Cloning Facility in Castle Oblivion, a Riku Replica (they might be able to make another one), Ventus Sora or Vanitas. I personally am betting on Vanitas because how else do you explain Sora's Heartless (a lot of people say Sora's Heartless was the one that attacked Destiny Islands in the Original Kingdom Hearts) and let's face facts if he could have come to Destiny Islands if he followed Ventus. He could also have replaced Sora's original Heartless and he has a striking resemblance to Sora. I hope this answers your question.