An imperfect replica of Roxas created from Sora's fragmented memories Xion which allows her to wield a Keyblade. She can be viewed as an anti hero in the game as she's neither good or bad just a loss tragic soul. Her existence stops the process of Sora's memories coming back which puts this project on halt. Her looks resemble Kairi as that's Sora's strongest memories though during the game her physical appearances change depending who she's talking to. Originally her face was blank until Sora's memories came through that gave her an identity.
The first week after Roxas joins the Organization the new member, Xion, comes at first she's very shy to Roxas during the first few missions but eventually opens up to him. Xion becomes close friends with Roxas and Axel as the three eat ice cream everyday during sunset after their missions. Later Xion loses to a battle against Riku and distances herself from everyone so Xion can discover who she really is. She discovers that she is not a Nobody, but a "puppet" created by Xemnas from the scattered memories of Sora to serve as a similar role of Kairi. As Naminé tries to restore Sora's memories it affects Roxas and Xion as well, preventing Sora from regaining his memories.
While this is happening her powers begin to become stronger while Roxas inversely grows weaker. This makes Xion want to escape the Organization. Axel and Roxas are ordered to track Xion down as she escapes so the friends are forced to fight each other. Eventually, Axel allows Xion to escape peacefully however, Roxas loses his trust in Axel as this makes him upset. At Twilight Town Xion attacks Roxas trying to absorb his powers to become Sora. Xion is defeated requesting her final wish before she dies that Roxas must release Kingdom Hearts and stop Xemnas's plan before merging with Sora as one, causing everyone's memories of her to vanish as if she never existed.