Every person in this universe is born with a Heart, a Soul, and a Body, regardless of if they were born in the Realm of Darkness or the Realm of Light. However, mysterious events have begun creating strange monstrosities; thus, those people who remain whole are now called Somebodies. As the natural inhabitants of the Realms, only they can wield Keyblades.
Strange forces have arisen in the Realm of Darkness, monsters that consume other beings’ Hearts to further their power; however, they are mindless, only seeking the acquisition of more Hearts. This endless desire has led some to infer that they are lacking a Heart, and so they were named the Heartless by both realms. However, further study has shown that they are merely just disembodied Hearts, utterly consumed by Darkness, and now seek to consume other Hearts and so propagate themselves: Hearts taken by the Heartless also become Heartless.
The Heart is the essence of a person, granting emotions and personality. Thus, when a Heart is stolen, the Body and Soul left behind simply wither away and die, with no drive left. However, those rare few with strong Hearts can, through sheer willpower, continue their existence, coexisting with their Heartless. However, without a Heart, their life is empty of emotion and so meaning, and their bodies degrade into twisted mockeries of Somebodies. These empty shells, something so clearly alien to Somebodies, became known as Nobodies. However, as Nobodies become increasingly common, the most powerful ones seem to retain their drive, and nearly remain identical to their former appearance as Somebodies; indeed, some have become even more powerful because of the loss of their Heart. Lacking Hearts, they now desire above all else to find their Heart and become Somebodies once more – no matter the cost.
As the Keyblade War has continued to rage and twist the worlds, even stranger creatures have appeared: the Reversed. People in the Realm of Light are born with Hearts filled with Light, and those from the Realm of Darkness with Hearts full of Darkness. Yet people have begun disappearing, only to reappear later completely changed, the alignment of their Hearts reversed, completely twisting them. As the very antithesis of what it means to be an inhabitant of their home Realm, yet still alien to the other, the Reversed by nature seek to corrupt and destroy. Their powers tap into the negative emotions of people and strengthen them, creating bizarre creatures. Unlike the Reversed, who are more or less accustomed to their twisted nature, the creatures spawned by the Reversed are not accustomed to this twisted existence: they are those Unversed in their own existence.