Basically like I said earlier a Nobody is what's left over by people whose hearts have been consumed by the Heartless. 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. Those with the strongest wills manage to retain human forms with some slight changes to their appearance. Others...not so much. Therefore, the more humanoid a Nobody, the more powerful it is. This is opposite the nature of the Heartless, whose form is more monstrous the stronger the original heart is.

All Nobodies are born in a world in-between the realms of light and darkness. 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 lack emotion. On the other hand, Nobodies manage to keep their memories of their former lives. However, even though Nobodies don't have emotions, they can think and plan, unlike Heartless who just attack.

Of course, there are some unusual Nobodies that almost have real emotions but they also don't have the memories of their Other. It's all complicated.