―Xion's final words to Roxas before he forgets her completely.
Xion is the Rank XIV member of the original Organization XIII, and appears in Kingdom Hearts: 358/2 Days as one of the main protagonists and main antagonists (albeit unwillingly). Xion is only a nominal member of the Organization, and does not have a throne in Where Nothing Gathers, nor does she have a title or a breed of Nobody to control.
She is not a normal Nobody; she is an imperfect Replica of Roxas created from Sora's leaked memories as a fail-safe in case both Sora and Roxas proved to be useless to Organization XIII's plans. While this enables her to use the Keyblade, Xion's existence also prevents 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), Xigbar sees her as Ventus, Xemnas sees her as Sora, while Saïx (and also Axel, initially) sees her only as a hooded doll. As Naminé stated while talking with Riku, Xion's face was originally blank, until Sora's memories found their way into Xion and she gained an identity.
Xion's Organization coat is similar to the one Roxas wears, with slightly pointed shoulders and closer-fitting sleeves. She also wears the more feminine, heeled boots that Larxene wears and the basic black gloves. Physically, Xion strongly resembles Kairi in her appearance from Kingdom Hearts, having her blue eyes and virtually the same haircut, though Xion's hair is black and her bangs part on a different side. She is slightly shorter than Roxas. As a result of her being crafted from Sora's memories, Xion's appearance is somewhat unstable, as she is known to transform into Roxas (in his Black Cloak) and both Riku and Sora in their original appearances from the first game. Her appearance is also slightly different depending on who is looking at her. Most of the Organization, such as Saïx and, early on, Axel, view her as a hooded puppet, while to Xigbar she appears identical to Ventus. Xemnas, on the other hand, sees her true form — a perfect copy of Sora.
First arriving at Organization XIII, Xion is a shy, secluded individual. When going on a mission with Roxas, Xion begins to open up to him, and later Axel. She does seem to express emotions due to her nature of existence, and is always happy to be with her friends. However, she is somewhat emotionally fragile, becoming deeply enraged with the right provocation. After finding out the origins of her existence, she also becomes deeply confused and lost. She has a strong sense of justice and generosity, seen in the way she willingly gave up herself to join with Sora. She resembles Kairi and Naminé in the way that she fights to protect those she has come to care for.
Xion seems to take an interest in seashells. While Roxas was in a coma when Sora was put to sleep, Xion placed a seashell on his bedside table every day he was asleep. In her nightmare on Destiny Islands, Roxas is in the dream and picks up a shell while whispering her name. After Xion was absorbed into Roxas, all that was left of her was a single seashell. This is likely a reference to how Kairi also had a liking for seashells too.
As an indirect Replica of Sora, Xion's fighting style as a playable character is very similar to his. After absorbing many of Sora's memories using the Organization XIII Devices, Xion is able to replicate Sora's Limits from Kingdom Hearts. However, Xion's movement is quite similar to Limit Form's, jumping back and stepping forward between attacks and slashing through the enemy in the finisher. Like Roxas, she thrusts with one hand and her finisher resembles the Vortex ability. Xion's Limit Break, Event Horizon, involves covering her Keyblade in an aura of Light and performing a devastating combo. Xion can also call down elegantly shaped pillars of light to attack nearby and far away enemies. Before Xion obtains the Keyblade herself, she is limited to simply shooting magic out of her hand, none of which is Light-based.
Once she obtains her Keyblade, Xion proves adept at handling it, being able to fight with it using her right or left hand. Her boss battle depicts her using many powerful melee attacks she seemed to have inherited from Sora. She also uses light much more often during her boss fight, such as creating spiraling circles of Light that suck in the player, leaving them vulnerable to one of Xion's aforementioned physical attacks. She can also use a Light spell similar to Ragnarok, and can envelope her weapon in Light to make her other attacks even more powerful.
Xion's Limit Break in Kingdom Hearts 358/2 Days is Event Horizon (イベントホライズン Ibento Horaizun?). During Event Horizon, Xion rapidly darts around the field with her Keyblade enveloped in light, slashing away at enemies with great strength. In her Final Limit, she summons four thin pillars of light to quickly move in straight paths to ends of the field, dealing heavy damage to any enemies in their path.
As Roxas's Replica, Xion can wield the Keyblade. She wields her weapon fairly skillfully, being able to disarm Axel. Because Xion and Roxas are both part of Sora, they can both wield his Keyblades, even at the same time. Because their weapons are the same, the Gears produce the same forms when applied to either Keyblade.
It is notable that, when Xion runs with her Keyblade, she grasps it with two hands instead of slinging it over her shoulder like Roxas does. This is identical to how Sora runs with his Keyblade in Kingdom Hearts II.