a theory..

[Sunset_Horizons]

I'm pretty sure that the "Lingering Spirit" or "Mysterious Man" that you fight in Kingdom Hearts II Final Mix + is Xehanort. Not the Xehanort, but the very same Xehanort who was Ansem's apprentice, known as Ansem, Seeker of the Darkness and/or Xemnas.

Anyway, despite all the visual footage implied in Birth By Sleep, which is clearly unnecessary but always helpful, Xemnas sought out the "Room of Sleep" and the "Room of Awakening", to find out his "friends", according to Xigbar. Now, we see that when Xemnas reaches the "Room of Sleep", he speaks to an armor of a knight, previously seen in Sunset Horizons with two other knights. Xigbar makes it very clear that is looking for the other "friend", which means, that not only does Xemnas have a past relation with them, but also that he is only seeking two of them. Why ? Very simple. He was one of them. The so called, "Lingering Spirit" or "Mysterious Man" who wishes deeply to engrave his hatred for Xehanort in Sora's heart, is none other than the original form of Xemnas. Of course his name wasn't probably Xehanort, it's something he claimed after he lost his memories.

So putting it simple, Nomura is doing the exact same thing he did in Kingdom Hearts I and II. Ansem, the Seeker of the Darkness was not the Ansem, the Wise, but rather his apprentice, Xehanort. Xehanort will turn out not to be the Xehanort, but rather this "Mysterious Man", whose name is not yet known. And once again, we get to taste a little bit of Xehanort in another form, just as it had happened in Kingdom Hearts Final Mix, when the "Enigmatic Man" was in fact, Xehanort's Nobody.

 

[Jacko]

I have some ideas to what has been wondered about..

I think that the reason everybody thought his name Xehanort was because when he lost his memory the only thing he remembered. his deep hatred for the real Xehanort made it the only name to stay in his head when he lost his memories. if you had name buzzing around in your head and not being able to remember anything, you would probably think that it was your name.

Remember at the world that never was how Xemnas said "how long has it been since I abandoned that name" perhaps when he remembers everything, he remembers who Xehanort was and called him self Xemnas.

'The room of sleep' is at radiant garden Hollow Bastion...
'The room of awakenings' is at Castle Oblivion...
The Room of sleep is at Radiant Garden Right, Well thats where Xehanort was found. Maybe he lost his memories there... Remember how as he walks down to the room of sleep he remembers how Ansem said 'those thoughts are best forgotten' Room of sleep, Forgetting...
The room of sleep is at Castle Oblivion, Perhaps thats where he went to awaken his lost memories...

 

[Water Dancer]

But if the knight was the real Xehonart, wouldnt that mean when they meet they would fuse with each other and become one again like Sora and Roxas did?