Nobody

A Nobody is what is left behind by those whose hearts have been consumed by Heartless. The first generic Nobodies are seen in Kingdom Hearts II, although members of Organization XIII and Naminé appeared as early as Kingdom Hearts: Chain of Memories and Kingdom Hearts: Final Mix, but it was unknown they were Nobodies.

Nature
Nobodies are born when a heart is swallowed by darkness and becomes a Heartless. They are the remaining parts left behind by the heart: the body, giving a Nobody form, and the soul, giving the Nobody life. However, only those with strong wills such as Sora are actually able to continue on as Nobodies. Those with the strongest wills manage to retain human forms with some slight changes to their appearance, while the rest possess malformed, barely humanoid appearances. Therefore, the more humanoid a Nobody, the more powerful it is.

All Nobodies, vanishing from the realm of light upon the loss of the heart, are born in a world in-between the realms of light and darkness, like Twilight Town or Castle Oblivion. However, because of this and their lack of hearts, they are shunned by both light and darkness, and do not truly exist.

Because of their missing hearts, Nobodies lack emotion. Although, some humanoid Nobodies pretend to possess emotions, fool others as well as themselves. On the other hand, Nobodies manage to keep their memories of their former lives, but unusual Nobodies lack memories of their former selves. Such memories are the reason why they can pretend having emotions. However, even though Nobodies don't have emotions, they can think and plan (as opposed to the Heartless, which simply act on instinct, with a few exceptions). Although it's never stated, the events of the Battle of 1000 Heartless implied that Nobodies are stronger than Heartless, since the Nobodies were able easily overpower the Heartless when they fought.

Reaction Commands
Reversal Reaction Command available enemy: Dusk, Xemnas's lighting strike.
 * Info: You swing around a Dusk, so that you are facing its back. This also works on Creepers if one is standing nearby, and is used in the battles with Twilight Thorn and Xemnas (fourth incarnation).

Berserk Reaction Command available enemy: Berserker, Saix
 * Info: The weapon of a Berserker is grabbed. Sora can then use it against them and cannot take damage while holding it.

Fail-Safe Reaction Command available enemy: Assassin
 * Info: Instantly kills an Assassin

Warp Snipe Reaction Command available enemy: Sniper, Xigbar, while trying to snipe you in the tower.
 * Info: Sora warps to defy gravity and send a Sniper's bullet back at them.

Learn Reaction Command available enemy: Dragoon,Xaldin
 * Info: The Attack command changes to Jump, which allows Sora to jump above the enemy's head, then pierce down the enemy targeted with the Keyblade's edge facing downwards. A total of nine Jumps can be stocked.

Stop Dice/ Begin Game Reaction command available enemy: Gambler, Luxord
 * Info: Sora must hit a certain command to defeat a Gambler. If he gets it wrong he is turned into a playing card or dice.

Duel Stance Reaction command available enemy: Samurai, Roxas (Final Mix+ only.)
 * Info: Sora enters a one-on-one battle with a Samurai, if he wins by selecting the right command it does damage but if he loses he will take damage. While fighting with Roxas, if Sora selects the right command he will knock away Roxas's Keyblades, which allows Sora to utilize them and all of Roxas's attacks are limited on using light crosses only.

Types
Bold text = Boss Italic text = Non-enemies
 * Dusk
 * Berserker
 * Sorcerer
 * Samurai
 * Sniper
 * Assassin
 * Dancer
 * Dragoon
 * Gambler
 * Creeper
 * Twilight Thorn
 * Organization XIII
 * Naminé
 * Scepter (With Marluxia)