Okay so... I really don't believe that Player possessed Ephemer. We never get the slightest inkling that he/she is upset and betrayed at not having been told about the keyblade war. We can see from the very beginning, till the end (of Union x, that is) that Player is a good person, who doesn't hold things against people. Someone who goes out of his/her way to help everyone he/she can. I mean, have you not been to all those worlds? Just can't leave a person in trouble, that kid.
And besides, how would Player 'just possess' anyone in that state at the end? He/she just magically transports to Ephemer (AND to the future, without help from the lifeboat or anything) and manages to gain the powers to possess someone, even when he/she didn't have the powers of darkness in the first place that would allow him/her to do such a thing? A longshot, if you ask me. And just to clear another thing thing up: we know that Ephemer had already transported to the future and to an unknown location, while Player stayed back. There's no way he/she managed to possess him before he left. Not to mention, again, the lack of motivation and means to do so.
But I do believe that Player is Xehanort, and I'm actually surprised no one's pointed out this very simple fact yet: Player was told by Chirithy at the end that if he/she chose not to sleep, then their heart will melt into a NEW one (not a used one - in other words: no possession). We literally see scenes right afterwards of a BABY Xehanort (not possessing anyone, not Ephemer, just a BABY), who then grows into someone we know very well. So yeah, new person, new life, but who obviously also has memories of his ("his" specifically now because Xehanort) 'past' self - because that person, Player, IS actually him.
Even if I turn out to be wrong, I'm really surprised no one's pointed this out. I'm not usually the one who makes a hundred theories, but even this one seemed pretty obvious to me. And again, even if it's not true, it's still a very logical explanation (as logical as KH scenarios can go).
...But I'm like 99% it's true.
(LOL reading my post while proof-reading made me realise how aggressive I may have sounded. Unintentional, sorry. Just got a bit passionate, I guess).
