Heartless are the darkness in hearts of living beings given shape and form. They have a thirst for hearts, and are most attracted to pure hearts, such as Sora's. Their existence became known after one was created by Ansem, as revealed by his reports. Emblem Heartless were later created by Xehanort. They first appear in Kingdom Hearts.
Though the origin of the Heartless is not explicitly described or shown, there was one point where there was not a single person with darkness in their heart, and only light existed, and as such all the worlds were connected. However, people began to grow selfish over the light, and thus darkness was born in their hearts. This darkness eventually consumed the light, but the children of light were able to craft new worlds, though they would remain separate. However, the greatest light would reside in the darkness, and until the day comes when that light breaks through the darkness, darkness would continue to thrive.
An unknown, but large, amount of time later, Ansem the Wise, the ruler of Radiant Garden, discovered the threat of the darkness. In order to combat it, he and his six apprentices, Xehanort, Braig, Dilan, Even, Aeleus, and Ienzo, tried to find a way to prevent the darkness from consuming the world, performing many experiments on the human heart. However, Ansem the Wise began to see the immorality of what he was doing, and ceased his experiments.
But Xehanort and the others continued with their experiments, discovering the Heartless and their nature. They constructed a large factory that could reproduce the creation of Heartless, thus making what were dubbed as Emblem Heartless. This resulted in the Heartless, once a minor threat, turning into a force that threatened all the worlds.
Becoming A Heartless
Although the experience of becoming a Heartless is relatively unknown, there is some depth of feeling when Sora is transformed into a Heartless in Hollow Bastion near the end of Kingdom Hearts. The best way to describe it is as a never ending fall in a completely pitch black area. The rest is unknown since the cutscene is short and the player resumes to play as a Shadow.
Pureblood Heartless are the natural Heartless, born when people's hearts are normally consumed by darkness. They are ink-black in color, with yellow beady eyes. They are more common in places that are close to or saturated in darkness. Upon their destruction, they simply disappear in puffs of darkness.
Emblem Heartless, on the other hand, are created from machines that reproduce the process of a heart being consumed by darkness. They have more varied shapes, colors, and are branded with Xehanort's symbol to differentiate them from the Pureblood ones. They mostly appear in the many worlds that reside in the Realm of Light, unlike naturally-born Heartless.
Heartless, Emblem or Pureblood, possess several key characteristics that define them as Heartless. All Heartless are created when the darkness that resides in a person's heart consumes him or her, thus giving the darkness shape and form (this process occasionally forms Nobodies, which are born from the body and soul left behind when the heart is lost). Being born from darkness, they are mindless and master-less, and their only goal is to find hearts, wishing to consume them and consequently create more Heartless. But what they desire above all are the hearts of worlds, and thus they enter worlds in search of the way into the hearts of the worlds. When they consume these colossal hearts, the remains of the world form new worlds, such as Traverse Town and the End of the World.
At the same time, Heartless seek out the users of the Keyblade, since the Keyblades are able to vanquish Heartless very effectively. In order to stop the users of the Keyblades, the Heartless use the Keyblade itself as a homing beacon, but ultimately desire to take the heart that commands it. Because of this, Keyblade wielders are under constant attack.
Heartless unlike Nobodies have hearts which means they do not act strategetically and as intellectually as Nobodies. Instead the Heartless rely on pure insinct. Nobodies on the other hand, have no hearts, which means; no hearts, no instincts. They only have brains.
Heartless, being formed from darkness, have varying strengths, depending on the amount of darkness in a person's heart. The darker the heart forming a Heartless, Emblem or Pureblood, the more powerful and more monstrous the resulting being will be. An exception to this is Xehanort's Heartless, who retained his human appearance for unknown reasons. This is a direct contrast to Nobodies, which determine rank according to the strength of the heart, and whose appearance becomes more human as they increase in power.
The Heartless, being mindless, usually have no master to direct their actions and instead act on the instinct to gather more hearts. However, there are some higher beings who can command them. Heartless will obey those with an affinity for darkness and a strong will. If many people attempt to control them, the Heartless will side with whomever is most powerful. Those with hearts, such as Maleficent, endanger themselves however, as the Heartless are attracted to their hearts. If they show weakness, the Heartless will not think twice of consuming their hearts.
On the other hand, higher ranking Heartless, like Xehanort's Heartless, can fully command other Heartless without endangering themselves. Also, Organization XIII is able to command them as well with their strong wills, but are not endangered of being consumed by them for their lack of a heart.
Heartless make use of the "corridors of darkness", inter-dimensional pathways that connect the many worlds. These pathways are located in the Realm of Darkness, and thus are very dangerous to use if one is not accustomed to the darkness.
The Heartless reside wherever darkness is abundant, especially the Realm of Darkness. However, they also reside in places such as the End of the World, a collection of the remnants of worlds that is located near the darkness. Another area that the Heartless live in is The World That Never Was, the world of the Nobodies that rests dangerously close to the darkness upon which the Heartless thrive.
Heartless, as they seek and consume hearts, also desire to return to the greatest heart, the heart of all worlds, Kingdom Hearts, that resides deep within the Realm of Darkness. And because of this, Heartless seek immense darkness, enough to completely consume the Realm of Light. The entryway into this place of great power is the Door to Darkness, the door that separates the realm of light and the realm of darkness. However, with the efforts of Sora and the King, the door is closed, which causes the worlds lost to the darkness to be restored, and preventing a massive army of Heartless from pouring into the realm of light.
The forces of the Heartless were extremely weakened after the door to Kingdom Hearts was sealed off, though there were still many left. At the end of Kingdom Hearts II, it is unknown what exactly happened to the Heartless and if they still pose a threat. It should be noted though that Yen Sid has mentioned that the only true way to destroy all Heartless is to have all people have light filled in their hearts and no darkness, leaving nothing to create a Heartless or something for them to thrive for.
- Although Purebood Heartless are born of people losing their hearts and not experiments, a resident of Traverse Town can be seen turning into a Soldier Heartless, which is an Emblem Heartless.