The Concepts of Reality
The Three Components of a Being
In the world of Kingdom Hearts (the overall world, that is, not just the individual "worlds"), every living, sentient creature and person is what's known as a "Somebody." Be they good, evil, or whatever else, if they're not a Heartless or a Nobody, then they're a Somebody. The being of a Somebody is made up of three primary components: the Body, the Soul, and the Heart.
The Body is a Somebody's physical being, their presence in the tangible world. The Soul is a Somebody's mind or will. The Soul is what contains things like memories, impulses, or instincts. Finally, the Heart is debatably the most important part of a Somebody. The Heart contains emotions and drives, as well as a Somebody's inner light and darkness. Every Heart, both good and evil, has positive and negative feelings, which means there is a wellspring of both light and darkness within it, respectively.
Typically, a Somebody can't exist without all three of these components. In the case of natural death, for instance, that being the destruction of a Somebody's body, their Soul and Heart are sent in tandem to the Final World, after which one can't really say what happens. However, there are exceptions to this, brought about by means both magical and mundane. It is from these means that the Heartless and Nobodies arise.
What is a Heartless?
A Somebody's Heart can be completely consumed by darkness, either from their own inner wellspring as a result of a negative emotional overload or from an external source of darkness. Either way, when this happens, their Body and Soul are erased, leaving a disembodied Heart swimming in a pool of Darkness. This disembodied Heart takes shape in the form of a Heartless, a creature of darkness.
The Heartless are driven by an instinctive desire to spread their darkness wherever they can by stealing the Hearts of Somebodies. This is also why Heartless are instinctively drawn to wielders of the Keyblade; Keyblade wielders' Hearts shine bright with both light and darkness, serving as a beacon of sorts for hungry Heartless.
As creatures driven by the darkness, the Heartless are instinctively subservient to those with powerful wellsprings of Darkness in their Hearts. Of course, attempting to control the Heartless in this way is extremely dangerous, as losing control of your emotions will drive the Heartless into a Heart-snatching frenzy.
Heartless born from natural sources of darkness, like one's inner darkness or the Realm of Darkness, are known as Pureblood Heartless. These kinds of Heartless have pure black or purple bodies, and in the case of larger Purebloods like Invisibles or Darksides, a large heart-shaped hole in their chests. Meanwhile, Heartless born from artificial means like dark magic are known as Emblem Heartless, named after the red and black Heartless emblem stamped somewhere on their bodies. These Heartless take on stranger, more varied forms, often reminiscent of the World their Somebodies came from.
What is a Nobody?
As previously mentioned, when a Heart falls to darkness and a Heartless is created, the Body and Soul simply fizzle out of existence. However, when a Somebody possesses an exceptionally strong Soul, driven by powerful convictions and will, the Body can continue to persist on its own as an independent entity. This is how a Nobody comes into being, an ephemeral creature that is not meant to exist.
As creatures of neither light nor darkness, Nobodies can freely traverse through both realms with little concern for their safety. They don't have Hearts that can fall to darkness, after all. Unfortunately, in spite of this perk, the transition to a Nobody is not generally a positive one. Most Nobodies undergo severe physical mutations and mental distortions, assuming bizarre, ephemeral forms and exhibiting erratic behaviors.
Unlike the Heartless, as Nobodies still possess their Souls, they are capable of sapient thought. Nobodies can plan, strategize, and coordinate with one another to get what they want. However, they do still have a base instinct they can't escape: Nobodies almost constantly long for their Hearts back, constantly adrift in an emotionless void of being.
Without their Hearts, Nobodies can't actually feel emotions, only reacting to situations based on their memories of how they would have reacted if they still had them. With enough time and emotional stimulation, it is technically possible for a Nobody to develop an entirely new Heart, but this is a rare occurrence.
What are the Unversed?
The Unversed have been described as the opposite of human life. They are physical constructs of raw negative emotion and, much like the Heartless, they seek to corrupt and devastate any worlds they come across. The Unversed are named as such because they are unversed in their own existence, hence, the opposite of life.
As to how they come to be, it is when one experiences powerful negative emotions that exist inside nearly every Heart that then intermingle with a powerful source of raw darkness that comes directly from the Dark World. Then take on the form of an object from the world they are from, adapt it to their own, then become a creature formed from the object in question.
What are Dream Eaters?
Dream Eaters are beings that exist and are born in the Realm of Sleep. They can also be created artificially. They are the Realm of Sleep's own personal way of darkness taking form as creatures. They are also the physical embodiments of dreams. They consume dreams. There are two types of Dream Eaters: Spirits and Nightmares. Spirits eat bad dreams, while Nightmares eat good dreams and plant bad ones. When they are destroyed, they seem to simply vanish, but Dream Eaters essentially work the same way dreams do since they are the physical embodiments of dreams, so all you'd have to do to get a Dream Eater to return after being destroyed is to dream it up again. That would be pretty difficult though since you can't really control what you dream about.
Certain Wielders in the past used Dream Eaters as companions and faced a terrible ordeal together. When all was said and done, the Spirits merged the Heart of their Wielder partner into themselves and then safeguard it while the Wielder goes to "sleep" . While they are not common in the waking world, there have been sightings and places among the Worlds where these wayward beings reside, looking for their lost pals.
What are the Devoid?
When a world falls to Darkness and winds up being ultimately consumed and destroyed by it, it is because the Heartless discovered a World's Heart through a Keyhole; a secret location to Keyblade Wielders to keep locked up and away from the Heartless. However, in the cases where a World's Heart is shattered, from it come the combined raw emotions of each of that world's inhabitants and from them, new beings are born. The smaller shards form creatures that absorb emotion and detritus to give themselves a body and head out into other worlds. The larger ones become sapient and conscious individuals whose wills and conviction strongly resonate with their destroyed world's. A world cannot become whole again until all their World Heart shards are collected into one and said shards connect with the World and the combined Heart must be willing to return to the world, otherwise, the most powerful shard will assume control and become far stronger and uncontrollable.
