This was all taken from another website's Video Game board without permission. But it makes so much sense, I simply had to post it.
Also, this may have been posted before, I dunno, but I'll post it anyway, because I'm ******ed.
"I knew it, that young guy IS Xehanort. Just after Xehanort took his body.. which could also explain the memory lost.
In the ansem reports it said that Xehanort said his body was too old. The bald dude! He takes tall knight's body and the guy loses his memory. Ansem the Wise finds him, heals him, and teaches him. He betrays Ansem the Wise, takes on Ansem's name and starts all the bull**** in KH 1 and 2."
"1. These 3 fight against Xehanort. Xehanort is too old to continue on his mission so he needs a new body. He takes the tall knight's body.
2. Tall knight loses his memories. Ansem the Wise finds him and brings him back to health. He learns from Ansem the Wise as he grows up.
3. When he's older he betrays Ansem the Wise. He takes on Ansem's name and gives in to the darkness. His Heartless becomes Ansem in KH 1/2 and his nobody becomes Xenmas in KH II.
4. His heartless tries to return all worlds to darkness. His nobody tries to destroy all worlds and create a new one with Kingdom Hearts."
