Xion

Yes, the untapped power that lies within you. Now, child, it's time you awakened that power and realized your full potential.
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"Who am I?"

—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 incomplete replica of Sora created by Xemnas made from Sora's leaked memories. Her face was based off of Sora's strongest memory, Kairi, but as Naminé pieced together Sora's memories, she began to physically look like Sora.

The plan was for Xion to eventually become a complete Sora replica, due to the fact that Xemnas was unsure if he could rely on the actual Sora to gather him enough hearts for Kingdom Hearts. Eventually she would have had to absorb Roxas to become whole, and if that were to happen, the original Sora would never awaken. Eventually she is absorbed by Roxas instead, thus giving him the ability to duel-wield.

Story

Kingdom Hearts 358/2 Days

Xion is first introduced in the game on the Seventh Day wearing her hood. After Axel is sent away to Castle Oblivion ,Xion and Roxas are sent on missions together. On one of their missions we discover Xion can use the Keyblade. After some time, Roxas and Xion begin eating ice cream together on top of the Clock Tower. On one of their missions at Beast Castle, Xion says she cannot summon the Kingdom Key on her own, so Roxas let's her borrow his. After defeating the boss, Xion gives Roxas back his Keyblade saying she no longer needs it, and summons the Keyblade on her own.
One day Xion goes missing, and then we see her the next day crying on her bed. We are then shown a brief flashback of her fight against Riku. The next day Xion avoids Roxas and does not go to the clock tower. The next day Roxas has a mission in Twilight Town and sees Xion on top of the clock tower. He goes up to talk to her but she seems depressed by something. Roxas says that they should complete the mission together,she agrees. After the mission they go back to the clock tower for more ice-cream. Xion finally has a full flash back on her fight against Riku.

Xion collapses in pain, she is holding the Kingdom Key.
Riku closes in on her and lifts her hood, she resists.
He lifts one eye of the blindfold to see her face.
Riku’s expression changes to surprise.
Riku: “Who are you? Why do you have the keyblade?”
Xion: “Why are you dressed like a member of the Organization?”
Riku walks away from her, towards the castle.
Riku: “So no one interferes with Sora’s sleep.”
Her keyblade is sticking out of the ground.
Riku picks it up and hurls it toward her.
Riku: “What you have… That keyblade must be a fake.”
Xion: You say this keyblade is a fake…? Don’t say such a selfish thing!
Xion moves to grab the abandoned keyblade.
Xion: Hyaa–!
Riku avoids her attack and deals a blow to her side.
Xion: Augh…
Riku: Here’s some friendly advice, get away from those guy’s base immediately.
Riku: If you do, you can be together with those two people.
Riku starts to walk away.
Xion: “Don’t tell me that! You’re deceiving too!”
Riku: “Indeed, maybe I’m the one who doesn’t exist.”
Xion then screams and the flashback is over.

• This is an incomplete summary*

Personality

Xion is shown to be a kind and good friend, but she gets depressed after finding what she really is. To that extent, she eventually goes insane and wishes to become whole, and willingly tries to kill/absorb Roxas to make it happen.