Nobody

A Nobody (ノーバディ) 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, but it was unknown they were Nobodies. They are made of body and soul.

Most Nobody types are named after job classes in Final Fantasy, except for Dusks and Creepers.

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. This is opposite the nature of the Heartless, whose form is more monstrous the stronger the original heart is (with the exception of Xehanort's Heartless, who retained human form because he gave into the darkness willingly).

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, fooling 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 to have emotions. However, even though Nobodies don't have emotions, they can think and plan (as opposed to the Heartless, which are mindless and simply act on instinct, with a few exceptions), and possess memories of their past lives despite their lack of hearts. Nobodies also seem to be able to experience physical sensations, such as pain, as they negatively react to taking damage, and Naminé passed out from the pain of being impaled on Marluxia's thorns (this was seen only in the Chain of Memories manga).

Although never stated, the events of the Battle of 1000 Heartless implied that Nobodies are stronger than Heartless, since the Nobodies were able to easily overpower the Heartless. At the very least they are just as powerful, and the fact that they are much more intelligent than Heartless only heightens the threat they pose.

However, it seems that unusual Nobodies (like Roxas and Naminé) possess true emotions. This may be because they co-existed with their original selves (the Keyblade Master of the Realm of Light and a Princess of Heart, respectively) and held half of their essence within them, thus making them more human than other Nobodies.

Reaction Commands
Reversal Reaction Command available enemy: Dusk, Xemnas's & Twilight Thorn'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, Saïx
 * Info: The weapon of a Berserker is grabbed. Sora can then use it against them and cannot take damage while holding it. This reaction command continues on to several upward hits with the hammer, and then a flurry of midair attacks christened "Magna Storm."

Fail-Safe Reaction Command available enemy: Assassin
 * Info: Instantly kills an Assassin by forcing them underground before exploding, rather than having them explode above ground and damage Sora.

Warp Snipe Reaction Command available enemy: Sniper, Xigbar, while trying to snipe you in the tower.
 * Info: Sora teleports and defies gravity by standing sideways in midair and sends 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.  This name is a reference to the skill of the same name attributed to Dragoons in Final Fantasy.

Stop Dice/ Begin Game Reaction command available enemy: Gambler, Luxord
 * Info: Sora enters a gambling game with a Gambler. He must hit a certain command at the right moment to win. 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.

Common

 * Dusk - Kingdom Hearts II
 * Creeper - Kingdom Hearts II
 * Dragoon - Kingdom Hearts II
 * Assassin - Kingdom Hearts II
 * Samurai - Kingdom Hearts II
 * Sniper - Kingdom Hearts II
 * Dancer - Kingdom Hearts II
 * Berserker - Kingdom Hearts II
 * Gambler - Kingdom Hearts II
 * Sorcerer - Kingdom Hearts II

Organization XIII

 * Xemnas - Kingdom Hearts II (Playable Character in 358/2 Days)
 * Xemnas (Armor) - Kingdom Hearts II
 * Xemnas (Dragon) - Kingdom Hearts II
 * Xigbar - Kingdom Hearts II (Playable Character in 358/2 Days)
 * Xaldin - Kingdom Hearts II (Playable Character in 358/2 Days)
 * Vexen - Kingdom Hearts: Chain of Memories (Playable Character in 358/2 Days)
 * Lexaeus - Kingdom Hearts: Chain of Memories (Playable Character in 358/2 Days)
 * Zexion - Kingdom Hearts: Chain of Memories (Playable Character in 358/2 Days)
 * Saïx - Kingdom Hearts II (Playable Character in 358/2 Days)
 * Axel - Kingdom Hearts: Chain of Memories (Playable Character in 358/2 Days and Ally in Kingdom Hearts II)
 * Demyx - Kingdom Hearts II (Playable Character in 358/2 Days)
 * Luxord - Kingdom Hearts II (Playable Character in 358/2 Days)
 * Marluxia - Kingdom Hearts: Chain of Memories (Playable Character in 358/2 Days)
 * Marluxia's Spectre - Kingdom Hearts: Chain of Memories
 * Marluxia's Angelic - Kingdom Hearts Re:Chain of Memories
 * Larxene - Kingdom Hearts: Chain of Memories (Playable Character in 358/2 Days)
 * Roxas - Kingdom Hearts 358/2 Days (Playable Character in Kingdom Hearts II and 358/2 Days)

Other

 * Naminé - Kingdom Hearts: Chain of Memories
 * Twilight Thorn - Kingdom Hearts II
 * Absent Silhouette Vexen - Kingdom Hearts II: Final Mix+
 * Absent Silhouette Lexaeus - Kingdom Hearts II: Final Mix+
 * Absent Silhouette Zexion - Kingdom Hearts II: Final Mix+
 * Absent Silhouette Marluxia - Kingdom Hearts II: Final Mix+
 * Absent Silhouette Larxene - Kingdom Hearts II: Final Mix+

Trivia

 * While the Symbol of the Nobodies appears mostly in Kingdom Hearts II, its first appearance is in Chain of Memories. It appears beside the names of the various sleights the Organization members and the Riku Replica use, as well as on Marluxia's Spectre form.
 * In the clip Another Side, Another Story, after Roxas is seen asking on-screen "Where's Sora?", the words "The third enemy" appear among several other quotes, possibly referring to the Nobodies.