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 released 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.
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.
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, Mission 71, 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 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.
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.
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 a Nobody is destroyed it fades back into darkness. If their Hearts are released first then they become the original person they were born from as shown when Ansem was defeated first and then Xemnas, recreating Xehanort.
Role in the series
The Nobodies serve Fagin due to the fact that Organization XIII wouldn't go along with Mirage's loyalty policy, at first. Though so far they have yet to appear.