Why does xion have a keyblade in 358 2 days

Because,she is a puppit made to copy roxas


She's not really a nobody, she is a puppet who was created by the Organization XIII to steal the memories of Roxas in order to become the new keyblade wielder. It was a plan as a safe guard in case Roxas proved to be a failure or decided to go rogue. Roxas is not entirely a nobody, he is a nobody with a heart, and that heart is Ventus'. Ventus and Roxas are the same person, except with Sora's memories.

People wonder why did Sora lost his memories in the first place, this question hasn't been answered. To my theory, Naminee was the one who scattered Sora's memories when she became a nobody while the Organization forced her to. She was later rescued by Ansem the Wise and decided to collect Sora's memories back.