MastaofBitches said:
You are remembering that Nobodies are just what's left over when a person becomes a heartless, right? Nobodies like the Organisation are what's left over when somebody with an incredible amount of will becomes a Hearltess.
As for who else could survive the world falling into darkness... Well, Shirou and Kirei are probably the only two Master's who could survive, but the rest of the Nasuverse...
I don't think there's many, if any at all, who could survive.
Yes, I'm fully aware that a Nobody is simply the Body and Soul (life-force and "will") left behind when the heart is lost, that has sufficient will to keep moving around, and in the case of the very strongest, keep their human form with minimal changes. I should have worded that better.
The thing is, when a World's Heart is taken from behind the World's Keyhole by the Heartless, the whole World (minus fragments that form both Traverse Town and The End of The World) is sent to the Realm of Darkness. This is shown in the BBS Final Mix Secret Ending,
A fragmentary passage, when Aqua, trapped in the Realm of Darkness, finds the Castle of Dreams (the world of Cinderella.)
When this happens, the inhabitants of the fallen world who have not had their Hearts already taken by the invading Heartless have several possible fates:
One: They are lucky enough to enter one of the Corridors of Darkness that open just before the World's destruction and they will wander these until either their Hearts weaken too much, at which point they'll become a Heartless, or they'll exit them in a world in the Realm of Light (Sora used one of these to wind up in Traverse Town, and Riku used one to emerge in Hollow Bastion, although this may have been influenced by Ansem, Seeker of Darkness, as most survivors of worlds lost to the Heartless find their way to Traverse Town.)
Two: They are not lucky enough to find an open Dark Corridor, and begin to fade into Darkness with their Worlds. If they absolutely refuse to give into their inner Darkness due to being good-hearted, and therefore having enough light in their hearts, and are strong-hearted/willed enough to resist the Darkness completely, they lose their body (and probably their Life-force) and become a summon gem. As explained by the Fairy Godmother, the summon gems are the crystallized forms of strong, good-hearted denizens that did not fall victim to the darkness during the destruction of their worlds, and so are able to be called into battle for a limited time. Fairy Godmother, KH1: "This little creature lived in a world that was consumed by darkness. When a world vanishes, so do its inhabitants. But this one had such a strong heart, he became a gem instead of vanishing with his world." Despite Saber having B-class luck, she won't make it to a CoD, and will become a summon gem.
(Note that Sora used the Keyblade of people's hearts on himself, and therefore couldn't resist his own darkness, as it was unlocked. In theory, had he not escaped Destiny Islands when he did, he could have become a summon gem.)
Three: If they are not good-hearted enough to become a summon gem, but are still strong-hearted enough to not be swallowed by the darkness, they'll wind up in the Realm of Darkness in one piece. (Although Master Aqua, King Mickey, Riku (after A,SoD's defeat) and DiZ are all whole in the RoD at times, they are not part of this rule, as they all travelled there via different means.) However, as the Realm of Darkness is essentially the pureblood Heartless's home, and is full of powerful pureblood Heartless, it's not likely they'll stay that way for long.) In the event that a whole person in the RoD is defeated by a pureblood Heartless, then the same fate awaits them as those in:
Four: Someone who gives into darkness, or is defeated by a Heartless, will become a Heartless themselves, with said Heartless's strength being dependant on how much darkness they had in their hearts. If they have sufficient will, they'll become a Nobody too, which may even retain a human form if they have a seriously strong will.
It doesn't matter whether you're in the Realm of Light or the Realm of Darkness when your Heart is taken by a Heartless, your Nobody still ends up in the Realm of Inbetween. I figure that Shirou can't be the only determinator in F/SN, Tsukihime and MTNY. I'm planning on having Waver, or to be more specific, Lord El-Melloi II, become a Nobody minor Spoiler
due to him remembering that Rider ordered him to live, and the memory of that promise giving him the strength to not fade into the Darkness. /minor spoiler.