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.

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.

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.

Fighting Style
Xion