Okay, I saw that some people were interested in my theory (and some people may be a little sceptical), but it's just a thaught.
All right big battle at the end of Birth by Sleep (yes, this is part of the climactic final battle; give me some time to find where i read it and i'll show you).
YouTube - Kingdom Hearts II Final Mix - Secret Movie
Any way, this game is definately going to leave off a HUGE cliff hanger (setting up the other games). As you can see, Ven is injured greatly. Terra and Aqua are probably forced to leave him to stop Master Xehanort, and for some reason are unable to return. Ven is found by Ansem the Wise without a memory. All he knows is the name "Xehanort":
Secret Ansem Report 1 My efforts these many years have come to fruitation, with the world I govern having become a paradise worthy of being called Radiant Garden. Nurtured by the pure water that is the source of life, fragrant flowers bloom in abundance, and the people face each day with hopeful smiles. But where there is light, darkness always lurks. As noted in my earlier reports, I must solve the mystery of this darkness of the heart. This paradise depends on it.
I shall perform and experiment to probe the depths of a persons heart. One of my own apprentices, Xehanort, has volunteered to be a subject. The young man has served me ever since I nursed him back from deaths door some years ago. He had lost all his memories at the time, but later showed remarkable intellectual curiosity and readily absorbed my teachings, gaining deep wisdom. Any mental immaturity is surely due to his young age.
If I explore Xehanorts heart with psychological tests, I may be able to recall the past locked away within. My apprentice, Even, has also shown great interest in Xehanorts memories. But is he really the right subject? Xehanort does indeed exhibit extraordinary talents
Too extraordinary
Perhaps they are even superhuman.