Xion is the fourteenth member of Organization XIII, first appearing in the game Kingdom Hearts 358/2 Days, in which she could be viewed as both a protagonist and antagonist. She is not actually a Nobody, and not even considered a true member of the Organization. Because of this she does not have a throne, nor does she have a title or a breed of Nobody to control. Instead, she's 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 halts 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 Saix sees her only the hooded doll she started out as.
Xion's name contains the Japanese words for "tide" (潮, shio?), fitting the oceanic naming theme of Kairi and Naminé, and Aster tataricus (紫苑, shion?). According to Japanese floriography, Aster tataricus is used to mean "I won't forget you" or "remembrance". It is also revealed within the Secret Reports that her "original name" was "No.i", her ID as an experiment of the Replica Program. The "i" used here is in fact the imaginary number.
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. 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 to Sora being put to sleep to regain his memories. 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 doll, and begins seeing her as the black-haired girl that Roxas sees.
However, 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 begins 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, who questions who she is and why she wields the Keyblade, causing her to become torn between staying with her friends and leaving the Organization to merge with her true self as Riku suggests.
Xion travels with Riku for several days before being captured by Axel and brought back into the Organization. However 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. Eventually, after being tricked into fighting Roxas by Saïx, Xion decides to take Riku's advice and leaves the Organization.
After her departure, Xion comes into contact with Naminé at Twilight Town's Old Mansion, questioning her own existence and the fate that will soon befall Roxas. She discovers that she is not a Nobody, but an imperfect Replica of Sora created as a fail-safe in case Sora and Roxas turned out to be useless to the Organization's plans. Naminé also reveals that as a memory being, once Xion returns to Sora everyone's memories of her will inevitably be forgotten. Furthermore, Naminé's attempts to restore Sora's memories not only affect Roxas, but Xion as well, stating why they both keep having memory relapses throughout the game. Xion accepts the facts, and decides to go back to Sora. Before she does, though, DiZ barges in with the news that an Organization member has followed her, in which Xion says she will take care of it, despite Naminé's protests, and heads out of the mansion.
It is Axel who has arrived, having finally tracked Xion down. Not only is Axel curious of Xion's motives, he is also given specific orders to capture her again, which would possibly convince Roxas to return to the Organization. Xion refuses, leaving Axel no choice but to accomplish his mission with full force, leading to a heated battle. Having defeated Xion, Axel brings her back to The World That Never Was, but soon faints from fatigue. Xemnas then steps out from the shadows and carries the unconscious Xion away to reprogram her to use the devices he had planted throughout the worlds.
Soon afterwards, 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. 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. When done, he doesn't seem to recall who she is, though she assures him that he'll be better off now. Cradling her in his arms, Roxas shares an emotional conversation with her, during which she explains to him that she had no regrets for meeting him or Axel, reveals the true colors of the Organization to him, and pleads with him not to let Xemnas win before dying peacefully in his arms. Saddened, Roxas suddenly remembers her name and holds Xion's hand as her body crystallizes and disintegrates into nothingness, leaving behind a single Thalassa Shell.
Though Naminé states that everyone's memories of Xion will vanish when 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 Xion. 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, before Roxas starts his path up the skyscraper. Through him, Xion is able to throw the Oblivion Keyblade to Riku in order to tell him to stop Roxas before he can reach Xemnas, because she believes Roxas is not strong enough to face him yet. In turn, touching the Oblivion causes Riku to recall flashbacks of the time he spent together with Xion, putting him in shock for a moment. The flashbacks then reoccur when he reaches the ground to face the Neoshadows, but this time turn blurred and grainier as they continue, until only film static, most likely meaning their total withdrawal from Riku's memories.
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.
After Aqua speaks with Ansem the Wise about Sora, Xion is sitting atop the Twilight Town clock tower and eating sea-salt ice cream with her friends (Roxas and Axel). Then, Xion and Roxas say Sora's name while watching the sunset together.
Xion appears during the opening sequence, where she points at Roxas with her Kingdom Key. The scene then changes to another scene, where Xion is seen having fun with Roxas and Axel on top of the Twilight Town clocktower. The scene changes back and Xion vanishes into fragments of light.
Xion then appears to Sora in the World That Never Was. Sora takes a grip of Naminé's wrist, but then Naminé turns into Xion. Sora then sheds a tear (most likely because of Roxas inside of him) and wonders who the girl is. Xion then pulls away from Sora and runs away from him.
Xion later appears on Destiny Islands inside Sora's heart. She is sitting on the Paopu Tree and Riku initially mistakes her to be Sora. Xion watches the sunset and asks Riku, "What do you want outta life?" Riku can then choose from three options like Sora did in the original Kingdom Hearts. Xion disappears after Riku gives her an answer.
Day 50: Strange FeelingEdit
Roxas woke up today. I think he was dreaming the whole time. Being with him feels strange. Familiar. Like listening to the waves crash up against the shore. But why?
Day 73: What Now?Edit
I lost the ability to use the Keyblade, and I have no idea why. I don't know what to do about it. About anything. It happened suddenly, after Roxas woke up. If I can't collect hearts anymore, the Organization has no use for me. This is terrible.
Day 75: Gotta Do SomethingEdit
Roxas has been coming along on my missions to help out, since I can't use the Keyblade. Axel is covering for me, so I should be safe for now. But what am I going to do if I never get the Keyblade back? I can't lie forever.
Day 95: My PastEdit
Neither Roxas or I have any memory of our human lives. I don't even remember my first days as a Nobody. Roxas said he was the same, but I'm not so sure. For me, it's a total blank. I can't even remember the day I met Roxas, or Axel. It's all a blur.
Day 149: He's the ShamEdit
That man wearing the Organization's coat - that jerk! He's too powerful. I can't beat him unless I get stronger. And I'm not a sham. I'M NOT. He is. He's the one pretending to be something he isn't!
Day 151: These DreamsEdit
I wasn't ready to see Roxas again. I can't help but compare us. We both use the Keyblade, but we're so different. Does he dream? It seems like that's all I do anymore. Last night I dreamt of the ocean. I was drowning.
Day 193: Dreams, or Memories?Edit
I finally wake up, just to pass out again? Why? Because I'm a failure? I dreamt the whole while I was out. It was something nostalgic, but what? Why can't I remember? It could be a key memory from my human life. All I can remember is the sound of waves, gentle, in the distance.
Day 225: Secrets About MeEdit
My body feels heavy, and my head aches. I'm too busy dreaming to get any rest from sleep. Axel knows something. He's hiding some secret. It must be in Castle Oblivion. That's where I was born.
Day 300: Us and the SunsetEdit
We watched the sun set today. The three of us. I didn't think we'd ever be able to do that again. It's like that dream I had - the one where I was by the shore with Roxas and Axel, watching the sun set over the water. I just want to stay with them. But how? What should I do?
Day 321: My PlaceEdit
Axel's always helping me out, so I just assumed he would again. How long has he known about me? Maybe since long before we met in Castle Oblivion. But he told me to think for myself. I was so happy. But now, as I get stronger, Roxas keeps getting weaker.
I shouldn't exist anymore. They say that Nobodies were never meant to exist, but I'm the real affront. But before I'm gone, I want to help Roxas and Axel. Even just a little. I think the boy I saw today is the same Sora from my memories.
Day 352: What I Must DoEdit
Riku gave me this time. Axel told me to spend it thinking for myself. The Organization is determined to erase either me or Roxas. They'll never allow us to coexist. 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.
Day 354: Where I BelongEdit
I wanted to see Roxas one more time, but couldn't. What would I say? Did I ever really exist? Even if not, I just hope that those times we spent watching the sun set were real. I know Naminé will tell me how to get back where I belong. I should hurry to see her.