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 Xemnas appeared as early as Kingdom Hearts, it was unknown that he was a Nobody. They are made of body and soul.

All Nobody types are named after job classes in the Final Fantasy video game series, except for Dusks, Creepers, and the Twilight Thorn.

Kingdom Hearts
Very few people know of the Nobodies around the time of the first game, as Organization XIII has not made any major "moves" towards achieving their goal. However, unknown to Sora and the rest of the characters, two Nobodies (Roxas and Naminé) were created during the story, when Sora stabbed himself to free his and Kairi's heart with the Keyblade of People's Hearts. In Kingdom Hearts Final Mix, Xemnas makes an appearance as a secret boss, in the room where Maleficent's dragon form was defeated, although it was not known that he was a Nobody.

Kingdom Hearts: Chain of Memories
Organization XIII begins its campaign towards creating Kingdom Hearts, simply known as the Organization. No lesser Nobodies are found in this game, the only ones being the human-like Nobodies: Naminé, Axel, Marluxia, Larxene, Vexen, Lexaeus, and Zexion. Axel and Naminé are the only two that survive this game.

Kingdom Hearts 358/2 Days
While the term "Nobody" was used very rarely in Kingdom Hearts 358/2 Days, two varieties of lesser Nobodies, Dusks and Samurais, appeared in endurance or pot-smashing missions such as Mission 10, Mission 33, and even appear to stop Roxas from escaping in Mission 91.

Kingdom Hearts 358/2 Days provided insight of Organization XIII's actions and explained their goals, showing instances of the development of the Replica Program, the formation of the synthetic Kingdom Hearts, and the search for the Chamber of Waking. Organization XIII the game's central focus, members Axel and Roxas both shared the pains of being Nobodies when their discussions brought them to realize and acknowledge the fact that they couldn't feel emotion, nor did they possess hearts.

Kingdom Hearts II
The first mention of a Nobody was at Mysterious Tower when Yen Sid talks to Sora, Donald Duck, and Goofy about the types of enemies he would encounter. Using magic "holograms", he shows the image of a regular Nobody, and then the human-like Nobodies of the rest of Organization XIII (Demyx, Xaldin, Xigbar, Luxord, Saïx, and Xemnas), the main antagonists of the game.

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 a strong will such as Sora are actually able to continue on as Nobodies. Those with the strongest wills manage to retain human forms (under special circumstances, sometimes the Nobody is formed with some slight changes to their appearance,) while the rest possess malformed, barely humanoid appearances; such examples as Nobodies like the Creeper and Dusk. 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 Ansem (Xehanort), who retained human form because he gave into the darkness willingly).

All Nobodies vanish from the Realm of Light upon their creation at the loss of a heart and the creation of a Heartless. They are born in a world in-between the Realms of Light and Darkness, such as 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. Despite their lack of emotions, Nobodies are able to think for themselves and attack with definite planning (as opposed to the Heartless, which are mindless and function on pure animal instinct). 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). Also, when someone turns into a Nobody, the shell that is left behind stops aging.

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 that 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 (Sora, the Keyblade Master of the Realm of Light and Kairi, 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." This move has some type of magnetic effect, causing any enemies close to Sora to be stuck to him, and caught in the attack.

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. It also damages nearby enemies and even other Assassins underground.

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. He can teleport and continue knocking the bullet back up to 3 times.

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, which will destroy the Nobody and cause a rainfall of munny. If he gets it wrong, he is turned into a playing card or rolling die.

Duel Stance Reaction command available enemy: Samurai, Roxas (Kingdom Hearts II Final Mix only)
 * Info: Sora enters a one-on-one battle with a Samurai, as they both stop attacking and stand motionless, blades at the ready. The command menu goes blank as time slows down. After a few seconds, the command The End appears randomly in one of the four command slots. If Sora can reach the command in time, he'll do a quick finishing slash that does damage, but if he's too slow, he will take damage from the Samurai's slash.
 * While fighting with Roxas, if Sora selects the right command, he will knock Roxas away and steal his Keyblades, which allows Sora to use them and Roxas's attacks are limited on using his light crosses only until he eventually acquires them back.

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 Nobody-like machine.
 * As stated by Nomura in an interview, when a Heartless gets destroyed by the Keyblade and it's heart joins with Kingdom Hearts and then it's Nobody's other dies, it makes them become whole again. However, when a Nobody is destroyed before it's Heartless' other, since it has nowhere to go, it disappears into nothing. This implies that the Organization XIII members, particularly Xehanort, could potentially return in future installments as their original selves, but it remains questionable as to whether Xehanort will return as his original incarnation (Master Xehanort) or his second incarnation (Xehanort).
 * It is implied by Axel that Nobodies can have their "type" changed or downgraded (likely by Xemnas), when he expresses that a possible repercussion of him disobeying the Organization (about bringing in or destroying Roxas) would be "getting turned into a Dusk".