Xion appears in Kingdom Hearts: 358/2 Days as the fourteenth member of Organization XIII. She joins the Organization shortly after Roxas. Like Roxas, she is able to wield the Keyblade, making her vital to the Organization's mission to collect hearts from the Heartless and complete Kingdom Hearts. She is also similar to Roxas in that she has no memories of her past life prior to becoming a Nobody.
Xion and Roxas form a friendly bond during the early portions of the game with some encouragement from Axel, who first became friends with them both. She routinely joins them to eat Sea-Salt Ice cream while sitting at the top of the clock tower in Twilight Town after completion of the daily missions organized by Saix.
Xion is playable in the cooperative multiplayer mode of 358/2 Days. Her base stats and abilities are identical to Roxas.
Xion's Origin and Fate
Xion is not an actual Nobody. She is a replica created from some of Sora's fragmented memories, specifically his memories of Kairi, thus explaining Xion's resemblance to her. Xemnas had hoped that utilizing both Xion and Roxas's Keyblade-wielding abilities would help accelerate the process of collecting hearts to construct Kingdom Hearts. However, Xion eventually learns the truth behind her creation, and her body becomes unstable. Coming to the realization that the memories she's comprised of should return to Sora, she forces Roxas into a final battle and is defeated, allowing her body to fragment and the memories to return.
Unfortunately, because Xion was artificially created from someone else's memories, all memory of her existence fades away with her. By the time the events of Kingdom Hearts II occur, no one remembers that she had existed at all.
The name Xion is an anagram of "No i" with the customary Organization "X" added. In this case, "No i" represents "No 1," or "No one."
As a Boss
As the second to last final boss of 358/2 Days, Xion has four different forms. Each form is related to a different world. Her final form is fought in front of the Twilight Town clock tower. Each of her boss forms are Level 42.