That's been CONFIRMED to be WRONG. The Knights are ALL NEW characters, as STATED by Nomura himself. It was confirmed in an interview. The Knights are characters we have NEVER seen before.
This is MY theory-
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Xehanort is the TRUE Keyblade Master of Darkness. When he realized the power he had he was slowly corrupted. He had an ally in a fellow TRUE Keyblade Master, The Master Of The Present, Ienzo.
Ienzo was the recent addition of Keyblade Masters, making four of them. Slowly though, over time, Xehanort's power grew even greater. And he struck down the Masters of Light and In-Between.
After finding the World Of Darkness, Xehanort stored the three Keyblades there, the Kingdom Key, The Way Of Dawn, and his own Inverted Key. He stored them there knowing only those Keyblades had the ability to stop his plan of controlling the worlds and becoming God-like. Deep within the Planet lies the Heart of all worlds. He would have to find it, but it was taking too much time.
Xehanort, having lost the ability to wield the true Keyblades, manipulated the Darkness to create countless Keyblades. Hoping to stall for time until he found the Heart.
Thus Began The Keyblade War. Ienzo was forced to fight against his friend and the war lasted for some time. Finally Ienzo met three Knights. The next ones in line who were to step up and become True Keyblade Masters.
Ienzo goes off to fight Xehanort alone, against the wishes of his allies. Then at the World Of Darkness, they would gather. The Knights had discovered that three TRUE Keyblades rested there. The World Of Darkness is where the Keyblade War would come to an end.
The Three Knights travel through a graveyard of false Keyblades, finally discovering the true Blades. Each one picks up a respected blade. Looking off into the distance, they see Ienzo coming towards them.
He arrives, but as an enemy. His heart had grown corrupt, whatever had happened when he fought against Xehanort turned him. Ienzo reasons with his allies. Telling them that the Heart of all Worlds has been found. That they can have real power.
But the Knights won't betray the honor of the Keyblade. It is then that they fight. Ienzo, still being able to wield his Keyblade, is BARELY beaten. The Knights believe Ienzo to be dead.
The Knights arrive on time to Find Xehanort. They make one last attempt to stop him and succeed. They then erase his memory, leaving him with only a name, and send him off into the Sea Of Worlds.
Using their Keyblades they create the door of Kingdom Hearts and lock the world and it's heart behind it. But inside, Ienzo is alive. Using the last of his strength, he gives his heart to darkness and arrives at Radiant Garden, but the price is high.
The World Of Darkness is then slowly consumed by the Darkness of the False Keyblades and it evolves into the world we see in the original game.
Ienzo can no longer wield his Keyblade, thus the Keyblade Of The Present has been lost. ironically enough, Xehanort also found his way to Radiant Garden, where Ansem The Wise has found him.
Ienzo, hearing the rumors of this man, goes to find him. He tries to allow his friend to remember, but he fails. Xehanort has no memory of the power he once had. Or of the truth.
Slowly, over the years, Ienzo managed to manipulate the Wise Ansem into allowing him to be taken as an Apprentice. Ansem does so, believing the boy to be quite intelligent.
Over time, Ienzo manipulates both Xehanort and Ansem into experimenting with the Darkness of the Hearts. Hoping to continue what his friend once attempted to do.
And so it begins again..............
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And that's what I think.