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I guess this is a completely hard question considering there is very very little information. I would guess that is someone related to Xehanort considering his dark Keyblade, which has features similar to the other Foretellers. I would go as far to assume his the player that went to darkness to which the dark chirity refereed to. Maybe he wasn't dark until that very moment, and its because of Xehanort going to the past and changing stuff in order to try to advance the process of him becoming part of the darkness.
Ansem seeker of darkness in KH1 said some peculiar words that didn't make since until now: You have come so far and yet you do not know nothing, you understand nothing. Peculiar phrase for an old guy, who simply manage to control a heartless and a Nobody of himself which in turn is from the body of someone else (Terra's body).
That's all I can think of... xD
The rules of time travel in KH exists specifically to keep MX from being able to just travel back to the Keyblade War.
There is a loophole with that theory though. If somebody was to use the Book of Prophecies to summon any incarnation of Xehanort to the time of X, the fact that incarnation is even present regardless if they were an illusion or not means that Xehanort can travel to that particular moment in time (albeit for as long as the projection summoned from the Book of Prophecies is present).
But back to topic, I fell as though the Sixth Apprentice would complete the Ephemera and Skuld trio for X. It gives the Sixth Apprentice a means to be relevant outside of their connection with the other Foretellers.
Considering the nature of the series I think it's safe to assume that even these illusions summoned from the book can have/grow a heart just like pretty much anything in this series.Do we even know how real these illusions summoned from the Book of Prophecies are? Do they have hearts? The time-travel rules clearly specify there must be a version of yourself in the past. Granted, an illusion of Master Xehanort could work, but time-travel works by discarding your body, and transferring your heart to a past time where it was. If illusions don't have hearts, then that technically doesn't work.
Considering the nature of the series I think it's safe to assume that even these illusions summoned from the book can have/grow a heart just like pretty much anything in this series.
Also, a heart may not even be a requirement to be considered a version of someone since YX clearly travelled in time to fetch Xemnas, and as far as we know he is consdering a 'reincarnation' of Xehanort like anybody else. (though one could argue that Xemnas might have Terra's heart but again, but it's not Xehanort's so who really knows what goes at this point)
We can also argue that Xemnas grew a heart of his own.