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, but 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. As Sora's memories are the basis 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. It is possible that she may have the same abilities to control the Samurai Nobodies.

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 one 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 in the bizzare room of Wonderland, Oogie's manor's ruins and the lamp chamber in the Cave of Wonders where you fight Jafar in the first game, 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, and she reveals the true colors of the Organization to him before dying peacefully in his arms. Emotionally shattered, Roxas absorbs Xion's degenerating body, 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.

It is Xion's death that compels Roxas to depart the Organization to find himself.

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 does display fits of rage, even towards her friends when they try to stop her from leaving. She is also very fragile, going insane when she discovers what she is.

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 suppose to exist?" "I want...I wanna be...With you two..." (talking to Roxas and Axel) "So, do you hate me for taking your friend away from you?" (to Riku) "Look at me Roxas. Who do you see?" (to Roxas)

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 appearence 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 female's after the transformation.