Nobodies (ノーバディ, Nōbadi) are bizarre creatures created from the remains of those who have lost their hearts to the Heartless in the Kingdom Hearts series. They are the second race to appear as the series' generic enemies after the Heartless, although they are not as diverse as their Heartless counterparts.
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 are actually able to continue on as Nobodies. Those with the strongest wills manage to retain human forms while the rest are barely humanoid and appear within otherwise deformed shapes. Therefore, the more humanoid the Nobody, the stronger its will.
Because of their missing hearts, Nobodies are said to be incapable of feeling emotions by every credible source and are shunned by both Light and Darkness, as they are not meant to truly exist. Some humanoid Nobodies pretend to possess emotions using the memories of their former lives, allowing them to act. Nobodies are also able to operate with intelligence and can feel physical sensations such as pain, unlike the Heartless which are completely instinctive. With their powers and resilience, they not only rival the Heartless in strength, but their ability to think and plan makes them all the more of a threat.
However, it seems that unusual Nobodies (like Roxas and Naminé) possess true emotions. Kingdom Hearts 3D: Dream Drop Distance proves this observation to be true in all Nobodies; Nobodies can indeed develop new hearts and emotions on their own, as was the case with Roxas, Axel, and Naminé. 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. However, the Heartless must be defeated first and the Nobody second in said order for the person to be restored.
Kingdom Hearts: Chain of Memories
Several Nobodies of Organization XIII mark the first appearance of the Nobodies in the series, where they coax Sora into entering Castle Oblivion by Marluxia, who attempts to manipulate Sora using a Nobody named Naminé, Kairi's Nobody, by telling her that helping them gives her purpose in life.
Unlike members in Kingdom Hearts II, these members of the Organization stationed in Castle Oblivion didn't use the services of lesser Nobodies against Sora or Riku.
Kingdom Hearts 358/2 Days
Continuing their plan to to create Kingdom Hearts, Organization XIII collects hearts through fighting the Heartless with the use of Roxas and Xion's Keyblades. The Organization members are not accompanied by lesser Nobodies during missions. However, Roxas faces Dusks and Samurais at The World That Never Was which reappear later on when they attempt to stop Roxas from leaving the Organization.
Kingdom Hearts II
After reawakening and entering Twilight Town, Sora, Donald, and Goofy encounter a group of Nobodies in front of the train station, but don't know what they are. When they meet Yen Sidat the Mysterious Tower, he explains the nature of Nobodies and Organization XIII while telling them of their new mission and gives them images of what the Nobodies look like. The remaining members of Organization XIII appear later in the game, and demonstrate different forms of Nobodies exclusive to each member, which Sora ends up clashing with. In The World That Never Was, the lesser Nobodies witness the collapse of the artificial Kingdom Hearts by the hands of Ansem the Wise, and try to catch the falling hearts, some of which spawn masses of Heartless that swarm the castle.