Xion is an imperfect replica of Sora, whose identity as a black-haired Kairi was eventually modeled from Sora's memories of Kairi.[2][3] She joined Organization XIII as the nominal XIV member and later joins as the XIII member of the real Organization XI
This is shown to be nothing but a ruse, in connection to her thoughts by her Secret Reports. She is aware that they cannot coexist, that Roxas was in danger if so, and decided to willingly disappear, and not let Xemnas succeed: a fact that she states when she later dies. ...