What Ethnic Woman said was basically right.
Xion is a replica...similar but not the same as the Riku Replica (Repliku) from KH CoM (both were initially "built" by Vexen).
In order to give "her" life, Xemnas went and sampled Sora's memories (unknown battle in KH 1 Final Mix), so Xion's core are Sora's memories, which are also the source of her power.
By forming own memories that belong only to her she however developed an own identity, a mind and consciousness, much like Roxas did as well.
Xion also has the "ability" to absorb memories...which is however not controllable by her.
By absorbing more and more of Sora's memories through Roxas, who has a special psychic link to Sora due to him being his nobody, Xion was on her way to in the end become a "second Sora" which would prevent the real Sora from ever waking up.
Xion absorbed also other memories though, some from Riku, maybe even some from Roxas himself or even some memories of Ventus.
So she created in fact another memory mess which most likely will have a sort of impact in the future...not to mention she is on Sora's "to save"-list as shown in the secret ending of Birth By Sleep.