Xion

Xion is a new member introduced in Kingdom Hearts 358/2 Days. Xion was first shown in trailers for the new game and has also made brief appearances in demonstrations. She was first believed to have been Naminé, but Nomura later informed the press that it was a different character.

Spoiler, click to viewclick to hide

Xion is an imperfect replica of Sora created by Xemnas in case Sora turned out to be useless to the Organization's 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 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 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 Sora's traits in conjunction with Naminé's restoration of Sora's memories. After learning of her origins from Riku, Xion resolves to return to her original self and leaves the Organization. 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. 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.

(Source: Kingdom Hearts Wikia)