Xion

Xion is the fourteenth member of Organization XIII, first appearing in the game Kingdom Hearts 358/2 Days. She is not actually a Nobody, and not even a true member of the Organization. Instead, she's an imperfect clone of Sora created from his leaked memories by Xemnas as a fail-safe in case Sora proved to be useless to Organization XIII's plans. This, in consequence, 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 and component of her being, Xion resembles Kairi, who is Sora's strongest memory.

Like Sora and Roxas, Xion can wield a Keyblade, the Kingdom Key, leading to a brief confrontation with Riku who questions why she can wield the Keyblade, and whether or not it is real.

Kingdom Hearts 358/2 Days
When Axel is sent to Castle Oblivion, Xemnas entrusts Roxas to look after Xion, who quickly bonds with him. She reveals her ability to wield the Keyblade to him, and looks after him after he falls into a coma for several weeks in conjunction to Sora being put to sleep to regain his memories. After discovering that all Organization members in Castle Oblivion except Axel have been slain by Sora and Riku, Roxas, Axel, and Xion all begin spending more time together as per Roxas's suggestion and become fast friends.

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; 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, becoming torn between staying with her friends and leaving the Organization to merge with her true self as Riku suggests. Eventually Xion decides to leave the Organization, though becoming hostile towards Roxas and Axel when they attempt to stop her.

After her departure, Xion discovers that she is not a Nobody, but an imperfect replica of Sora created from his leaked memories by Xemnas as a fail-safe in case Sora turned out to be useless to the Organization's plans; the reason she resembles Kairi is because Sora's memories of her are the strongest ones he has. In addition, Naminé's attempts to restore Sora's memories not only affect Roxas, but Xion as well, to the point where she begins to take on Sora's physical appearance. This revelation drives Xion completely insane.

Xion confronts Roxas at the Twilight Town train station, and after revealing her altered form to him, transforms into an armored creature and attacks him, planning to kill and absorb Roxas in a delusional attempt to become a real person, uncaring that this would prevent the real Sora from ever awakening. After a grueling battle, Roxas defeats the deranged clone of his former self, who regains both her sanity and her original appearance. Cradling Xion 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, and reveals the true colors of the Organization to him before dying peacefully in his arms. Emotionally shattered, Roxas absorbs Xion's degenerating body (taking in Sora's memories), the energy granting him the use of her Keyblade (allowing him to dual-wield both his and hers as well) in addition to inadvertently bringing the restoration of Sora's memories to a complete stop.

In a Secret Report that you get around Roxas's Diary, it is revealed that she is experiment No. I of the Replica Project. Hence, the origin of her name.

Moments away for the end of the game, you see Riku and Roxas engaged in a battle and it seems Riku has flashbacks of meeting Xion at Beast Castle and Destiny Insland etc, which repeats a few times troughout the battle but each time it happens the scenes start to become blurry as though the memories of Xion 'No. I' faded away like they weren't real. It has been said that Namine was the one who did this, which would also explain why she was never mentioned in Kingdom Hearts 2 as everyone just seemed to forget about her. Also during the battle Xion speaks to Riku at some point about Roxas, as her last and final words.

Personality
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. While she may seem monotone, she does 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 discovering what she is, she begins developing signs of mental instability to the point where she loses all rational thought and goes mad.

Fighting Style
As a replica of Sora, she can wield the Keyblade and her fighting style greatly resembles his original Kingdom Hearts incarnation. However her style is more similar to Roxas. Like Roxas she thrusts with one hand and her finisher resembles the Vortex ability. She and Roxas can apparently share keychains with each other, allowing them to use the same Keyblades.

Quotes
"Is it that I'm not supposed to exist?" "I want...I wanna be...With you two..." "What would it take for me to be like you?" "So, do you hate me for taking your friend away from you?" "You have poured so many memories into me. Given me so much, that I feel like I'm about to over flow. Look at me, Roxas. Who do you see?"

Trivia

 * During Xion's battles, Roxas is transported to Wonderland, Halloween Town, Agrabah, and finally Twilight Town. It is unknown why Xion does this, though this is most likely a side effect of having Sora's memories, as said person has visited these worlds before.


 * When Xion takes the physical appearance of Sora, her Organization garments turn into a male equivalent (very similar to Roxas) which may imply that her entire body is the same as Sora's; however, her voice is still the same.


 * During her nightmare at Destiny Islands, Xion recalls not Sora's but Riku's memories of his confrontation with Zexion, something Sora never had the knowledge of. It is possible that some of the leaked memories that created Xion came also from Riku, however, that is never revealed.


 * During a scene where Xigbar is about to battle Xion in Wonderland, he sees her appearance as Ven, which distracts him, allowing her to knock him to the ground. This is likely due to the fact that she is a puppet based on Sora's memories, and emitted Xigbar's memories of Ven, but Ven's connection to Sora remains unknown.


 * Xion's character also appears to be related to sea shells. While Roxas was in a coma she placed various sea shells near his pillow and, after dying and being absorbed by Roxas, she leaves a single sea shell behind. It is also hinted that the sea shells came from Destiny Islands.