Nobodies are the prominent enemies of Kingdom Hearts 2, and the focus of the storylines of Kingdom Hearts: 358/2 Days and Chain of Memoires, which take place at around the same time.
Nobodies are beings that come into birth whenever someones whose Heart had become a Heartless is released. Only those with strong wills or exceptional values can be turned into both Heartless and Nobodies, otherwise just the Heartless will be released, the Nobody never coming into existence. Nobodies consist of both body and soul. There are two types of Nobodies: the Organization Nobodies, who are the most powerful Nobodies. The Organizatio Nobodies are Nobody versions of their original human forms, with an x embedded into their new names, whihc are amalgrams of their true names. They consist of thirteen members and control the lesser Nobodies. Typical Nobodies serve as shock troops and spies for the higher-ups, lacking conciousness and thinking skills. Typiclas are usually deformed and barely Human-looking, their power identified by how Human they look. Nobodies cannot experience emotions, per se. Yet they can fool their enemies by pretending to have them. Only specific Nobodies can feel emotion. (Roxas and Namine are great examples. Axel can, to a lower degree, experience something smilar to emotions due to his time spent with Roxas.)
Nobodies can also be changed in rank, their forms affected. This is proven when Axel implies of a punishemnt of being transformed into a Dusk.
If a Nobody is destroyed along with it's Heartless, the two halves are rejoined, the true being restored. However, if the Nobody is the first one to be destroyed, the soul will have nowhere to go; fading from existence and preventing resurrection.
Nobodies are made up of a third element of Kingdom Hearts: Nothingness. The Realm of Nothingness serving as the Game's final battlefield during the bout with Xemnas.
 Kingdom Hearts: Chain of Memories
The Organization members assigned to Castle Oblivion serve as the antagonists for Sora and Riku's trek through the Castle's memory-worlds. Marluxia, Axel, and Larxene serve as Sora's primary enemies (With a bout with Vexen) while Lexaeus, Vexen, and Zexion serves as Riku's obstacles.
 Kingdom Hearts: 358/2 days
While Sora and Riku make their way through Castlke Oblivion, Roxas is inducted into the Organization. Being Sora's Nobody, he can wield the Keyblde, much to the interest of his superiors, who use him to collect the Heart needed for Kingdom Hearts. These Nobodies serve as Roxas' main allies until the halfway point of the game, and all of them as Protagonist for the game's Mission Mode.
 Kingdom Hearts 2
The Nobodies again serves as the game's main enemy (Aside from the Heartless) The Nobodies and Heartless fight each other while Sora and company infiltrate The World That Never Was, the Nobodies' main stronghold. Their leader Xemnas, is eventually defeated by Sora and Riku, who are then returned to Destiny Islands after being stranded in the Real of Nothingness. It is unknown if Nobodies will appear in future installments of the KH series, but due to the destruction of their leader, this is most unlikely. However, their original selves may indeed apopoear, no official annoucement at this time.