Xion is the fourteenth member of Organization XIII and first appears in the game Kingdom Hearts 358/2 Days, in which she could be viewed as both a main protagonist and antagonist. She is not actually a Nobody, and not even a true member of the Organization. Instead, she's an imperfect Replica created from Sora's leaked memories as a fail-safe in case Sora or Roxas proved to be useless to Organization XIII's plans. While this enables her to use the Keyblade, Xion's existence also slows Sora's progression of regaining his memories to fully awaken while in his pod, something which Naminé, DiZ, and Riku recognize.

As Sora's memories are the basis of her being, Xion initially resembles Kairi, who was Sora's strongest memory at the time of her birth, though her physical appearance eventually changes depending on who is viewing her and who they have connections with. Roxas, Axel, and Riku see her as Xion (and later, as Sora), while Xemnas and Xigbar see her as Ventus, and Saïx sees her only as a hooded doll.