A Nobody is what remains of those who have lost their hearts to Darkness and have become Heartless. They are a "speices" of creature that appear in Kingdom Hearts, playing a major role in both Kingdom Hearts II, Kingdom Hearts: Chain of Memories, and Kingdom Hearts 358/2 Days.
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 (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.
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.
However, it seems that unusual Nobodies, like 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 and Kairi, respectively) and held half of their essence within them, thus making them more human than other Nobodies.
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.
Nobodies in Dead FantasyEdit
Naminé is the Nobody of Kairi. She was born in a different way from most Nobodies, as at the time of her creation Kairi's heart was within Sora. Naminé was born when Sora stabbed himself with a artificial Keyblade in order to return Kairi's heart to her body. This made Naminé a very strange Nobody, born from Kairi's heart and Sora's body and soul. Kairi's own body never became a Nobody because her heart lacked darkness, so her body and soul were never able to leave the Realm of Light and thus made it seem like Kairi was in a comatose state until she regained her heart.
This means that Naminé was born without Kairi's memories, only furthering her unique status as an unprecedented Nobody. Yet she and Kairi are incomplete without each other, similar to Sora and Roxas. Because of the fact that Naminé is formed from Sora's body, she has the power to manipulate his memories as well as the memories of those who are connected to him, prompting several others, even herself, to call her a "witch". Ansem the Wise stated in his ninth Secret Ansem Report that Naminé is a highly unusual Nobody, due to possessing neither the body nor the memories that a Nobody is usually made from. By the end of Kingdom Hearts II, Naminé was able to join Kairi so the two of them could become complete.
In Dead Fantasy, Naminé is still joined with Kairi within her, but in previews for the series, to shows that Kairi is either able to draw Naminé out, or allow her Nobody to take over in order to become more powerful. This action is shown by Kairi "physically turning into Naminé".
Dead or Alive - Kingdom Hearts - Final Fantasy - Ninja Gaiden