Xion, the only other girl in the Organization, aside from Larxene, is not really a Nobody, she was created from Sora's memories of Kairi which were stolen and modified by Naminé in CoM. That's why she looks like Kairi, and even their theme songs are similar. She can also use a keyblade in the start of 358/2 Days, but she loses this ability somehow, and is helped by Roxas in her missions. Each member in the Organization sees her differently, Roxas firstly saw her with the hood on, but once they became friends, he saw her without it, while the others still saw only the hood. Xigbar, who knew Ventus (they meet in Birth By Sleep), sees her with Ventus's armor.
She worries about Roxas a lot, especially when he's inconscious. Saix knows what she is, not a real Nobody, and mocks at her, calling her just a puppet, which makes Roxas really angry.
In the end, she runs away, because she found out that she was a part of someone else, Riku meets with her, in a mission she is given to eliminate him, and tells her what she is. She becomes terribly confused, but in the end, accepts her fate, to fuse back with Sora. She meets with Roxas, and he sees her face as Soras's instead of Xion's. then, they fight, and by the end of the battle Roxas no longer knows who she was, as he lost his memories of her, and he kills her, allowing her to be absorbed by Roxas. Right when she's dying, Roxas remembers her, and the death scene is terribly sad. She's the final boss in Kingdom Hearts: 358/2 Days, and has four armors, which change according to the place you're battling her.
Up to down, armors 1, 2, 3 and 4.
The last pic is when Roxas sees her as Sora, right before they fight.