Theories About Kingdom Hearts’ Master of Masters
He’s Riku’s Heartless
This is one of the wilder theories out there, but it at least has enough plausibility to make it worth talking about. The theory posits that Riku originally was born and grew up in the time before the Foretellers, which is what allowed his Heartless to travel back in time once it was created, as the rules of the KH Universe establish that you can time travel to a period if a past version of yourself existed there.
One piece of support for this theory comes down to Riku’s eye color, a piercing aqua blue, unlike anyone else’s eye color in the series. The only other eye color of this type is in the Gazing Eye in the Master’s No Name Keyblade, which also bears a striking resemblance to the eye in the hilt of Riku’s Way to Dawn Keyblade.
The idea is that Riku’s Heartless went back in time and became the Master, then put young/child Riku into the box to awaken at a later date. We know there has to be some form of putting people into sleep or preservation, because of Ventus’ presence in Union [cross].
Riku’s Heartless is known to be in Kingdom Hearts 3, so there’s a possibility he could be the one to transport everyone to the “Fated Place,” in this case the “Destiny Islands.” At this point, after the events of KH 3, he travels back in time and assumes the role of the Master, writing the book of prophecies, until he fades away.
It’s a bit of a crazy theory, but fun to think about nonetheless, and you can see a full explanation here.
