Nobody

A Nobody (ノーバディ) is what remains of those who have lost their hearts to Darkness. They are made of body and soul.

Kingdom Hearts
Very few people know of the Nobodies around the time of the first game. 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.

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

Kingdom Hearts 358/2 Days
Low- and High-ranking Nobodies do not frequently appear during Roxas's stay in the Organization; he faces Dusks and Samurai during three skill trials at The World That Never Was, Mission 10, Mission 33, and Mission 71, and then again when he finally makes his bid to leave the Organization.

There was provided insight of Organization XIII's actions and explained their goals, showing instances of the development of the Replica Program, the formation of the artificial Kingdom Hearts, and the search for the Chamber of Waking. Organization XIII 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, and Goofy about the types of enemies he would encounter. Using magic "holograms", he shows the image of the most common Nobody, the Dusk, 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.

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

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 the Nobody, the stronger its will. This is opposite the nature of the Heartless, whose form is more monstrous the stronger the original heart is (with the exception of Ansem, 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 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 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, seems to show a bond toward 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 return to nothingness; 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.

Nobodies actually can cultivate new hearts and develop emotions on their own, as was the case with Roxas, Axel, and Naminé.

Replica
A Replica (レプリカ) is an artificial copy of another being, and a special type of Nobody. The first Replica seen (though not the first produced) in the series is Riku Replica in Kingdom Hearts: Chain of Memories. Another Replica is later revealed to be Xion, Experiment No. i in the Organization's Replica Program. In the manga adaptation of Kingdom Hearts: Chain of Memories, Vexen makes a vast amount of Replicas of himself, which survive into Kingdom Hearts II.

Replicas (also referred to as "puppets" or "clones" by some other characters) are synthetic copies of other people. While the exact method of creating a Replica is unknown as of yet, they have been shown to be able to be created out of different materials. Riku Replica was created using data collected by Vexen from his fight with the real Riku. He believed he was the real Riku due to Naminé's memory manipulation.

Xion, an imperfect Replica of Roxas, was made by Xemnas from Sora's memories. Xion was so different from Roxas because the memories she was made from took the physical appearance of Sora's most important and strongest memory: Kairi. Replicas, like Nobodies, fade into Darkness after they are defeated. Xion, however, was absorbed by Roxas, therefore she continues to live on inside him, and inside Sora.

Most Replicas have a mysterious, stoic nature and very seldom give straight answers. Also, a trait shared by all Replicas shown in any version of the series is that they are very sensitive about the fact that they are copies. Riku Replica, upon learning he was a Replica, constantly referred to himself as "fake" and became obsessed with trying to become a different being than Riku. Xion even refers to herself as a "puppet" after she learns of her existence. In the manga, the Vexen Replicas are very quiet and sad compared to the original Vexen, telling the Riku Replica that "we are all failed replicas."

Replicas seem to have a few unique abilities. Replicas appear to be able to fight as soon as they are created, not needing to learn fighting techniques or become stronger. This is most likely a trait passed down from the person they were created from. The most prominent ability Replicas have is the ability to absorb the life-force of other beings, increasing the Replica's overall power and allowing them to use any unique abilities the being absorbed may have had. In fact, the main reason for Xion's creation was for her to absorb Roxas and become "the new Sora", thus preventing the real Sora from ever awakening from his sleep. Also, Riku Replica absorbed Zexion's energy in Kingdom Hearts Re:Chain of Memories, something that was encouraged by Axel. This indefinitely made Zexion fade as his power seeped out, and in turn, this made Riku Replica obtain more power.