=/ Just thought I'd point this out, as no one else did.
The affinity of what realm you reside in does not determine whether you are good or evil. We've seen many folks from the Realm of Light and where they all good little angels? No. In fact, I remember quite a few of them trying to kill the main character. Scar, Ursula, Oogie Boogie, Clayton, ect. They all were from and resided within the Realm of Light, yet they were not good. So saying that because it's Light it's automatically 'good' and Darkness is automatically 'bad' is ignorant on your part.
Seeing as we have Hades in the game with his little Underworld, it's quite possible that the afterlife in KH mirrors that of the ancient Greeks in which when you die, you are sent to Tartarus in which you are tried to determine what happens to your soul.
If you pass as a good and wholesome person, you get to go to a peaceful and pampered afterlife in which food and drink and other necessities are provided to you as desired.
If you're a bad or wicked person, you get your punishment decided and that's what you must do for all eternity (ie: Rolling an enormous boulder up an unreasonably high hill or having a vulture peck out and eat your spleen and have it painfully grow back overnight just to go through the torture all over again).
If you're not really either, then you remain in this one area that's neither good nor bad, just in the middle.
Bleh, I know there are specific names for each of these places and I probably botched all this information up, but it's been a while since I've studied the ancient Greek afterlife.