"I love Roxas and Axel. I'm sure Saïx would scoff at that. Call it a trick of my artificial memories. But the time I spent on that clock tower was real. I wish the three of us could stay together, just like this, forever. But I have to end this. I'll never forget today's sunset. Even if Roxas and Axel do, I won't forget."
—Xion in her diary
Kingdom Hearts 358/2 Days
Xion is the Rank XIV member of the original Organization XIII as well as a member of the real Organization XIII. She debuted in Kingdom Hearts 358/2 Days as the deuteragonist and later one of the final antagonists.
She is not a normal Nobody and the only a nominal member of the Organization. Because of this, she does not have a throne in Where Nothing Gathers, nor does she have a title or a type of Nobody to control. Instead, 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. Day 276: Behind the Truth Similar to Riku Replica, Xion was created by the Organization's scientist Vexen. While the nature of her existence enables her to use the Keyblade, Xion's existence also halts Sora's progression of regaining his memories to fully awaken while in his Memory Pod, something which Naminé, DiZ, and Riku recognize.
As Sora's memories are the basis of her being, Xion initially looked like a hooded doll, but her physical appearance eventually changes depending on who is viewing her and who they have connections with. Majority of the time, she resembles Kairi, who was Sora's strongest memory at the time of her birth, though Xigbar sees her as Ventus, and Xemnas sees her as Sora. As Naminé stated while talking with Riku, Xion's face was originally blank, until Sora's memories found their way into Xion while she gained an identity.
A week after Roxas joins Organization XIII, Xemnas introduces Xion to the other members. After looking at the other members, she smiles up at Roxas, puzzling him. As Roxas departs for his first training mission, he spots Xion shyly watching him nearby, and Saïx formally introduces her to Axel and Roxas.
When Axel is sent to Castle Oblivion, Xemnas entrusts Roxas with the task of looking after Xion, who quickly bonds with him. After only a couple of missions together, Xion removes her hood, surprising Roxas by her gender. She reveals her ability to wield the Keyblade to him and looks after him when he falls into a coma for several weeks in conjunction with Sora being put to sleep to regain his memories. Xion visits Destiny Islands, collects the seashells there including a Thalassa Shell and places them on Roxas's bed. When Roxas wakes up from his coma, Xion is happy and gives him a shell to listen to on the clock tower.
After discovering that Sora and Riku had slain all of the Organization members (except Axel) that were in Castle Oblivion at the time, Roxas, Axel, and Xion all begin spending more time together as per Roxas's suggestion and become fast friends, such as enjoying sea-salt ice cream together. After Xion asks Axel to be her friend, and he accepts, he stops seeing her as a hooded figure and begins seeing her as the black-haired girl that Roxas sees. Unfortunately, Xion can not summon the Keyblade, although she regains the ability on Day 96.
A series of events begin to split the trio apart: While DiZ and Naminé oversee the restoration of the sleeping Sora's memories (moving him from Castle Oblivion to Twilight Town), Roxas begins to experience the process for himself, and starts to ponder over how he is able to wield the Keyblade and doubt the Organization's motives; Axel is reprimanded by their superiors for placing his friendship with Roxas and Xion before the Organization's goals, and Xion is thrown into disarray after a confrontation with Riku at Beast's Castle, who questions who she is and why she wields the Keyblade, especially after he sees her face, although he lets her go.
On Day 150, she was reprimanded by Saix for failing to defeat an impostor which greatly upsets her. The next day she reveals to Roxas the dreams she's been having and her confusion on her role in the Organization.
On Day 171, Xion failed another mission, putting her in a coma. Roxas then begins giving seashells to Xion. After Xion has a dream, or more accurately, a flashback of Sora disappearing in front of Kairi at Hollow Bastion, Xion wakes up beside Roxas on Day 193.
On Day 225, Xion discovers the Organization's computer and reads records about her existence.
On Day 255, Xion visits Castle Oblivion and crosses paths with Axel who is on a mission. She learns who she is and escapes the Organization.
On Day 276, Xion decides to visit Destiny Islands again. She is overwhelmed by memories and faints. She is saved by Riku and he tells her of his current condition. He explains to her that his friend Sora needs Xion to return him his memories if he is to ever awaken. She doesn't understand why she has Sora's memories to begin with, causing her to become torn between staying with her friends in the Organization or leaving to merge with Sora as Riku suggests. Xion travels with Riku for several weeks, eventually leaving him to stay at DiZ's base in Castle Oblivion.
On Day 298, Xion is found by Roxas in Twilight Town and he invites her to return "home" with him, but she says she is unable to. Xion tries to flee again, but Roxas stops her, prompting her to point her Keyblade at him. Axel appears and fights Xion, knocking her unconscious, forcibly returning her into the Organization.
On Day 321, Naminé's attempts to restore Sora's memories are soon halted when Xion starts absorbing them faster, causing Roxas to weaken while she becomes stronger, as Xemnas intended. Later, Xion informs Axel of this strange situation and he tells her she could think of an answer to this problem herself. She also tells him she saw a boy today who looks just like Roxas, implying she saw Sora somehow.
On Day 352, after being tricked into fighting Roxas by Saïx at Halloween Town, Xion decides to take Riku's advice and leaves the Organization. The next day, Xion fights Xigbar in Wonderland who perceives her as Ventus and knocks him unconscious. She unhoods in front of Axel who was also on the mission, shocking him, and asks him to let her go, which he agrees to, and she retreats through a Corridor of Darkness. Xion talks to Riku at Beast's Castle, wondering what she should do next, and he implores her to find Naminé in Twilight Town for answers.
On Day 356, Xion meets with Naminé in Twilight Town's Old Mansion in order to learn about her nature and how it affects Roxas. Naminé informs her that she is a replica made from Roxas's memories, rather than a Nobody and that this means that reuniting with Sora will cause her to not only no longer exist, but also be completely forgotten. Xion persists in her commitment, certain that it's where she belongs but is interrupted by DiZ, intruding in the conversation to warn them that they have been located by a member of the Organization. Xion goes to investigate and finds Axel, upset that he has been tasked with retrieving her, and conversely that letting her reunite with Sora as she wants would also result in her termination. Xion explains that she is committed to her path, and asks Axel "Please don't hold back". In response, he promises that he will "always be there to bring [her] back". Axel is able to subdue Xion after an intense battle, and limps back to the castle with her unconscious body in tow, fainting at the entrance. From there, Xemnas drags Xion away to reprogram her using the Organization XIII Devices that he had planted throughout the world.
he next day, Roxas finds Xion at the Twilight Town train station, and after she reveals her altered, Sora-like form to him, she explains that she has been compelled by Xemnas to absorb Roxas and become a "full" Sora, which would, in turn, prevent the real Sora from ever waking. It is explained in her Secret Reports that this is somewhat of a ruse—she knows that Roxas will disappear if she continues to exist, so she plans to force Roxas to absorb her, saving himself and thwarting Xemnas's plan. She then transforms into a monstrous version of Sora using Xemnas's devices, and attacks. During the battle, Roxas and Xion are transported to Wonderland, Halloween Town, and Agrabah, before returning to Twilight Town. In each world, Xion absorbs power from the "devices" that were placed there, which contain memories of Sora's battles in each world.
After a long and grueling battle, Roxas manages to defeat her, while she regains her original appearance in his view. He doesn't seem to recall who she is, though she assures him that he'll be better off now. While Roxas cradles her in his arms, she tells him that she chose to sacrifice herself to slow down Xemnas's plans, asks that he prevent Xemnas from obtaining Kingdom Hearts, and tells him that she is glad that she met him and Axel. As Xion's body crystallizes and disintegrates, Roxas suddenly remembers her name and holds her hand until she disappears completely, leaving behind a single Thalassa Shell.
Though Naminé states that everyone's memories of Xion will vanish as she does, Roxas, Riku, Xemnas, and Saïx manage to retain some memories of her into the next day. By then, Roxas has returned to The World That Never Was in order to set Kingdom Hearts free, which he hopes will reunite him with his lost friend. However, he is being stalked by a swarm of Neoshadow Heartless, as well as a determined Riku, who has arrived to capture Roxas to finish Sora's memory restoration. Riku looks down upon the Nobody as Roxas starts his path up the skyscraper. When Xion's lingering influence makes Roxas throw his Oblivion Keyblade to Riku, she is able to communicate to Riku, begging him to stop Roxas before he can reach Xemnas, because she believes Roxas is not strong enough to face him yet. With the message, Riku receives memories of the time he spent with Xion, shocking him for a moment, but they begin to fade as he reaches the ground.
When Roxas and Riku finish eradicating the Heartless, they turn on each other for a battle. By this time, it is a struggle for Roxas to even remember Xion's name, and all he knows about her is that he wants her back. While Roxas is able to knock Riku down first, Xion then projects her words to Riku, somehow telepathically, to stop Roxas at any cost, and so he does once he removes his blindfold and transforms into Riku-Ansem. Unconscious, Roxas hears Xion talking to him through his mind, telling him not to be sad, as she has become one with him and Sora, and that soon Roxas will also join them. Despite the fact that memories of Xion have been forgotten by everyone, she tells him that she still has her own memories, so she doesn't mind that he has forgotten her, as she won't forget him or Axel. As Riku knocks him out, Roxas's last memories of Xion fade away.
Kingdom Hearts coded
In the Data Castle Oblivion, Data-Naminé reveals to Data-Sora and King Mickey that Xion is one of the people that, while gone, is still connected to Sora's heart. Data-Sora wonders who she is and remarks that she feels familiar and that something terrible must have happened.
Kingdom Hearts 3D: Dream Drop Distance
After Sora catches up to Naminé in his dream in The World That Never Was, she suddenly transforms into Xion. Xion does not respond to Sora's questions, merely looking at the floor with a look of sadness on her face. Although Sora doesn't recognize her, he suddenly sheds a tear, to his own surprise and confusion. Xion raises her hood and runs off.
After catching up to Xigbar, Sora comments on how Axel, Roxas, Naminé, and "that other girl" all must have had hearts. When Sora reaffirms to Xigbar that his strength comes from his friends, Xigbar sees an image of the many individuals connected to Sora standing behind him, including Xion. Unnerved, Xigbar leaves Sora to Xemnas and leaves.
After Riku dives into Sora's heart and defeats the Armored Ventus Nightmare, he appears on a memory of the Destiny Islands. After meeting Roxas and Ventus, who each ask him a question, Riku finds Xion sitting on the paopu tree. He does not remember her and questions who she is. She then asks him what he wishes, and disappears once he answers.
Between Kingdom Hearts 3D: Dream Drop Distance and Kingdom Hearts III
Saïx found Vexen's notes on replica No. i, sparking his memory of Xion; thus her revival was added to his atonement. While meeting at the Keyblade Graveyard with Master Xehanort, Saïx recommends reviving Xion to serve as Sora's replacement in their ranks.
Xion's heart is brought from the past by Young Xehanort and placed in a replica along with a fragment of Master Xehanort's heart. However, because her heart came from the time before she had a sense of self, Xion has no will to act; she is kept at the Keyblade Graveyard until the final battle.
Kingdom Hearts III
Lea’s subconscious memories of his time with Xion continue to persist as he trains with Kairi, and he often catches himself staring at her because their resemblance stirs his memories of his lost friend.
While traveling in the Gummi Ship, Sora and company are contacted by Ienzo, who informs them Sora has three unique hearts on his own. While they agree that the first two hearts belong to Roxas and Ventus, they are unable to identify Xion as the owner of the third.
On the eve of the final battle, Saïx tracks Lea down at the Clock Tower and points out that Lea has bought an extra ice cream bar for Xion without realizing it.
At the Keyblade Graveyard, Xion accompanies the real Organization when they confront the Guardians of Light and pairs with Saïx to fight Lea and Kairi. Saïx subtly tries jogging Xion’s memory by saying Axel and Roxas’s names. When Xemnas appears and attempts to kill Lea, Xion intervenes. Stirred by this, Roxas’s heart calls out to her via Sora, causing Xion to waver further, eventually leading to the present day version of her heart leaving Sora’s to inhabit the replica vessel and displace the past version back to its proper time. Shortly afterward, Xemnas’s imperilment of Xion and Sora triggered Roxas’s heart to fly to his waiting vessel, allowing him to stop Xemnas from executing her and Lea. Xion, Roxas, and Sora then battle and defeat Saïx.
After Master Xehanort opens Kingdom Hearts, Xion arrives with Lea, Roxas, Ventus, Aqua, and Terra to help King Mickey and Riku hold off its power while Sora sends Master Xehanort to Scala ad Caelum with the intent of keeping him from using Kingdom Hearts. She assures Sora that Kairi would be safe.
However, the Replica Xehanorts, which Sora had just fought, emerge from Kingdom Hearts and pull the Guardians into the sky where they are cornered by them. The guardians manage to triumph over the Replicas but end up getting sucked up into the multiple keyholes created from Kingdom Hearts.
Despite this, future Sora manages to free Xion and the others by connecting their bonds, allowing them to play a part in fully restoring Kairi to normal. Once Sora and Kairi defeat the Armored Xehanort replica, the Guardians of Light all rejoin at the battlefield to finish him off. Everyone aims their Keyblades at Xehanort and fire off a large beam of light that manages to pierce Xehanort, destroying him for good. Once the battle is over, the heroes all travel to where Sora's heart is located, which is Scala ad Caelum.
After Sora, Donald, and Goofy defeat Master Xehanort, Xion and the other Guardians help close Kingdom Hearts and return to the Keyblade Graveyard
In the aftermath, Xion, Roxas, and Lea return to Twilight Town at the clock tower to spend time together once again. Xion also becomes friends with Isa, Olette, Hayner, and Pence once they join the trio on the clock tower.
Riku arrives in the Highwind and takes them to the Destiny Islands for a party celebrating their victory over Xehanort. Xion is shown collecting seashells with Naminé and watches as Kairi shares a moment with Sora before he fades away.
After one year passes by, the Twilight Town crew are revealed to studying Roxas and Xion's memories in hopes of finding the whereabouts of the missing Sora.