Nobody

A Nobody (ノーバディ) is what remains of those who have lost their hearts to Darkness. The first generic Nobodies are seen in Kingdom Hearts II; although Xemnas appeared as early as Kingdom Hearts Final Mix, it was unknown that he was a Nobody. They are made up of body and soul. Depending how strong their human hearts were, the body and soul will take a human or monster form.

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

The Nobody emblem resembles a splintered, upside-down heart. It originates from Terra's memories of the Terra's Mark symbol, which are inherited by Xemnas. It is also present on Master Xehanort's Keyblade Armor. The symbol first appears in Kingdom Hearts: Chain of Memories, where it marks the names of sleights used by Organization members and the Riku Replica, as well as the head of the Specter.

Between Kingdom Hearts Birth by Sleep and Kingdom Hearts 358/2 Days
Xehanort's experiments culminated in his fellow apprentices' hearts and the creation of his own Heartless, Ansem, Seeker of Darkness, and Nobody, Xemnas. Together with Xigbar, the Nobody of Master Xehanort's co-conspirator, Braig, Xemnas founded Organization XIII, recruiting the Nobodies of the remaining apprentices as founding members. They recruited additional members and took command over the lesser Nobodies.

When Sora becomes a Heartless in Hollow Bastion while freeing Kairi's heart from his body, two Nobodies are born: Roxas appears in Twilight Town, where Xemnas recruits him as the thirteenth Organization member, while Naminé is born in Castle Oblivion, which the Organization has taken as a secondary base, and is taken into Organization custody.

During his second trip to Hollow Bastion, Sora encounters Xemnas in the room where Maleficent was defeated in her dragon form, and they fight before Xemnas disappears.

Kingdom Hearts: Chain of Memories
Sora is coaxed into entering Castle Oblivion by Marluxia, who attempts to manipulate Sora using Naminé's power over his memory. He and Larxene tell Naminé that if she inserts herself in Sora's memory, she can be "somebody" instead of just a Nobody.

Riku, who emerges from the Realm of Darkness in the basement of Castle Oblivion, encounters several Organization members, learning a bit about their nature as Nobodies along the way.

The members of the Organization stationed in Castle Oblivion don't use the services of lesser Nobodies against Sora or Riku, choosing to fight for themselves when not relying on the Heartless inhabiting the castle.

Kingdom Hearts 358/2 Days
Organization XIII continues its plan to create Kingdom Hearts, relying heavily on Roxas and Xion's Keyblades for collecting the necessary hearts.

The Organization members are not accompanied by lesser Nobodies during missions. However, Roxas faces Dusks and Samurais during three skill trials at The World That Never Was: Mission 10, Mission 33, and Mission 71. More Dusks and Samurais attempt to stop Roxas when he finally leaves the Organization during Mission 91.

Kingdom Hearts II
Roxas is accosted by many Nobodies attempting to retrieve him from within the simulated Twilight Town, as well as Axel.

Sora, Donald, and Goofy encounter a group of Nobodies in front of the Twilight Town train station, but don't know what they are. When they meet Yen Sid at the Mysterious Tower, he explains the nature of Nobodies and Organization XIII while telling them of their new mission. He shows them images of Dusks and hooded Organization members. Sora clashes with the Nobodies multiple times, including the remaining Organization members Demyx, Xaldin, Xigbar, Luxord, Saïx, and Xemnas.

Kingdom Hearts 3D: Dream Drop Distance
Within the Sleeping Worlds, Xemnas reveals to Sora that Nobodies actually can cultivate new hearts within their bodies and develop emotions on their own, as was the case with Roxas, Axel, Naminé, and Xion, a fact that was deliberately kept from the other Organization members as part of Master Xehanort's plans to use them as vessels for his heart.

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, the Nobody is formed with some slight changes in appearance), while the rest possess malformed, somewhat monstrous, barely humanoid appearances, such as with Creepers and Dusks. Nobodies become more humanoid the stronger their will.

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 are said to be incapable of feeling emotions by every credible source. 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. Through these memories, Nobodies are able to remember what it was like to have emotions, thus allowing them to respond correctly to specific situations. 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). The Kingdom Hearts: Chain of Memories manga and several other media forms provide an alternative view, saying that they do have emotions, but are plagued by a feeling of uncertainty and emptiness due to not having hearts. The manga further displays this "plagued" feeling by showing the members engaging in tasks like jigsaw puzzles, cooking, and reading to try to fill their emptiness.

Although never stated, the events of the Battle of the 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. Roxas and Naminé were born without any memories of their past lives due to the unusual nature of their births, and thus had no basis for any pretend emotions. Additionally, they co-existed with their original selves (Sora, the Keyblade Wielder 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. Although Axel denies the idea of feeling emotions, he shares a bond with Roxas and claims that Roxas makes him "almost feel like he has a heart" and has acted on his feelings of friendship a number of times.

When defeated, Nobodies are dragged into darkness; however, when a person's Heartless and Nobody both are defeated, that person is restored to their original, complete form, with their memories from their time as a Nobody intact. Lea, Ienzo, Dilan, and Aeleus all return to their human forms after their Nobodies are defeated; presumably their Heartless had been defeated at an earlier point.

It is implied more than once that high-ranking Nobodies can be reduced to a lower rank. Some members of Organization XIII consider being turned into a Dusk an implied disciplinary measure.

In Kingdom Hearts 3D: Dream Drop Distance, it is revealed that Nobodies actually can cultivate new hearts and develop emotions on their own over time, as was the case with Roxas, Axel, and Naminé, but Xemnas and Xigbar deliberately kept the other Organization members in the dark about this as part of Master Xehanort's plans.

Replica
A Replica (レプリカ) is an artificial copy of another being that can be considered a special type of Nobody. The first Replica that appears in the series is the Riku Replica, who debuts in Kingdom Hearts: Chain of Memories. Xion is later revealed to be another Replica, Experiment No. i in the Organization's Replica Program. In the manga adaptation of Kingdom Hearts: Chain of Memories, Vexen also creates a large number of Replicas of himself, who survive into the events of Kingdom Hearts II.

While the exact method of creating a Replica is unknown, a couple different methods have been demonstrated for the copying process, with different results. The Riku Replica was created using data collected by Vexen during his fight with the real Riku. Xion, an imperfect Replica of Roxas, was created by Xemnas from Sora's memories. Due to her imperfections, Xion's appearance varied in the eyes of different people, but because of Sora's powerful memories of Kairi, she resembled her to those that were closest to her.

Despite their nature as copies, Replicas have demonstrated the ability to develop personalities and a powerful sense of self. This creates a great deal of internal conflict when they discover what they truly are. When the Riku Replica discovers he is a copy of the real Riku, he becomes obsessed with proving himself to be more than a "fake," as well as for his existence to be his own. Xion, upon discovering what she truly is, starts referring to herself as a "puppet"; she is unsure of how to handle the revelation. In the manga, the Vexen Replicas are very quiet and sad compared to the original Vexen, telling the Riku Replica that they "are all failed replicas."

Replicas have unique abilities. They are capable of absorbing other individuals to increase their strength; in Kingdom Hearts Re:Chain of Memories, the Riku Replica absorbs Zexion's life-force to steal the Nobody's power, while Xion exists solely to absorb Roxas and become "the new Sora," thus preventing the real Sora from ever awakening from his slumber.

Replicas, like Nobodies, fade into Darkness after they are defeated. Xion, however, is absorbed by Roxas, therefore she continues to live on inside him, as well as inside Sora.

Non-Canon

 * Note: These are Nobodies that are not canon in the Kingdom Hearts Universe.