In Kingdom Hearts 358/2 Days, Xion is an individual who is recently introduced as a new member. You can see her in promotional materials for the game and she has made short cameos during demonstrations. At first, people thought she was Naminé, but later on, Nomura revealed that it was someone else entirely.
She then disappears, her scattered essence absorbed by Roxas, who gains the use of her Keyblade which results in his signature dual Keyblade fighting style.
Xion is an imperfect replica of Sora created by Xemnas in case Sora turned out to be useless to the Organizations plans. She was part of an intended series of replicas (her name being an anagram of her initial designation number, "No. i", based on the imaginary unit), though this series ultimately went unrealized. As Soras memories are the basis of her being, Xion resembles Kairi, who is Soras strongest memory.
After being captured by Axel, Xion is reprogrammed by Xemnas to absorb Roxas and become the "real" Sora, thus fulfilling her purpose. Xion approaches Roxas in Twilight Town and attacks him. After fighting, Roxas defeats her, and Xion returns to her senses. Xion tells Roxas that she is glad that they were friends and reveals the true intentions of the Organization, pleading with him not to let Xemnas have his way.
Xion is an imperfect replica of Sora created by Xemnas in case Sora turned out to be useless to the Organizations plans. She was part of an intended s
Like Sora and Roxas, Xion can wield a Keyblade leading to a brief confrontation with Riku. During her time with the Organization, Xion befriends Roxas and Axel, but eventually questions the nature of existence as she develops more of Soras traits in conjunction with Naminés restoration of Soras memories. After learning of her origins from Riku, Xion resolves to return to her original self and leaves the Organization.