Nobodies

A Nobody (ノーバディ Nōbadi?) is what remains of those who have lost their hearts to Darkness. The first generic Nobodies are seen in Kingdom Hearts II, when Yen Sid shows them to Sora, along with the left members of Organization XIII. They are made of body and soul.

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, Seeker of Darkness, 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.

Role in Ultima
In Ultima, with the destruction of Organization XIII and the revelation of Xemnas, Saix, and Xigbar's treachery to join the Organization, control over the Nobodies has been mostly passed down to the Circle of the Blot. Not only that, but thanks to the Shadow Blot and the Mad Doctor's experiments, new types of Nobodies have been created to serve in specific army positions for the Blot's Army. Of course, Xemnas and his fellow two nobodies, despite being vessels for Xehanort's heart and are still growing new hearts, are still heartless and thus can still command the Nobodies for their own purposes. In fact, Xemnas, despite being mostly Xehanort in personality and loyal to his master, is actually planning to confront the Shadow Blot to once and for all decide who is the true master of Nothingness.