In truth, she is just an artificial construct, ploy by Xemnas to manipulate Roxas into fulfilling his predetermined role. She's someone Roxas can relate to, a newbie like him; but like Namine, she's just a tool, not a real member. That's why she doesn't get a marker at Proof of Existence or the Garden of Assemblage, or even a merit addition to the name "Organization XIII".
Towards the end of the game, XIV disappears and Roxas learns that she was never hsi friend, that she was never really here, that he was just a tool all along, that the Organization would never give him the answers about Sora that he sought, and that even Axel (his best friend) was in on it the whole time. Feeling betrayed and alone, he leaves forever for parts unknown. Quoth he, "No one would miss me."
