Heartless

The Heartless (ハートレス) are beings of darkness which manifest in two forms, "Pureblood" and "Emblem". While most Heartless are in fact manifest hearts, they behave entirely devoid of emotion, and were thus named "Heartless".

Kingdom Hearts Birth by Sleep
Partly due to the fact that the common enemies in the game are the Unversed, not many Heartless appear in the game. During the time Ventus was still Master Xehanort's apprentice, he summoned several Neoshadows to fight him in an attempt to get the him to open his heart to the darkness and create the X-Blade, despite Ventus's pleas for mercy and cries that he was not strong enough. The Neoshadows overwhelmed him, forcing Master Xehanort to unlock his old apprentice's heart, creating a being out of the most pure, raw form of darkness, which he named Vanitas and took on as his new apprentice.

Much later, Aqua is sent to the Realm of Darkness following her battle with Terra-Xehanort. There, she encountered a pack of Darkside Heartless. Summoning Master Eraqus's Keyblade, she later decided fighting would be useless and let the Darksides come at her. However, both the Earthshaker and Wayward Wind came to her rescue, a last symbol of hope from her old friends Terra and Ventus.

The Heartless are not fightable enemies, only appearing in the two cutscenes afore mentioned. Heartless are only seen in Aqua's and Ventus's stories, and never mentioned or seen in Terra's.

Between Kingdom Hearts Birth by Sleep and Kingdom Hearts
Ansem the Wise suspected that his research was endangering the citizens of Radiant Garden, so he ordered his apprentices to cease the experiments on the hearts immediately. However, his foremost apprentice, Xehanort, continued the experiments. He also convinced Ansem's other apprentices, Braig, Dilan, Even, Aeleus, and Ienzo, to help him in his experiments on the heart, but eventually, the experiments drove them mad, and they descended to Darkness, then they revolted against their teacher, Ansem, and banished him to the Realm of Nothingness, where he was driven by revenge and became the mastermind, DiZ.

After experimenting further, they discovered the Heartless, who devour their hearts, as well as the citizens of Radiant Garden. The Heartless then began to thrive and spread to other worlds, devouring innocent people, and driving the worlds they invade into darkness. After a while, Maleficent makes the remains of Radiant Garden her new headquarters, renaming the twisted remains Hollow Bastion, and takes control of the Heartless.

Kingdom Hearts
The Heartless appear inside Sora's Awakening as preparation for the coming invasion of Destiny Islands. They succeed in separating Riku from the world, and then they steal Kairi's heart, they devoured Destiny Islands and everyone that lived on the islands, all except Sora, who slew many of them with the Keyblade. After he escaped, he met Leon and Yuffie, who told him about the Heartless, what creates them and what they hunger for, the Darkness in the heart of every living being. This leads Sora to fighting the Heartless alongside Donald Duck and Goofy in an effort to save the worlds.

The Heartless were actually being used by Xehanort's Heartless, who renamed himself as "Ansem, Seeker of Darkness", and Maleficent, who sought to use their power to rule over all worlds. However, Maleficent's plans were thwarted by Xehanort, who possessed Riku's body to pierce her body with a Keyblade formed from the Princesses Hearts, and change her into her dark ego. After she was beaten by Sora, Xehanort said that she was actually being used by the Heartless.

Sora rushes to where Kairi was, and then encounters Xehanort, who explained that Kairi's heart was inside of Sora the whole time. He also told his false name (Ansem) to Sora. Xehanort needed Kairi's heart so that the Heartless could devour the worlds, but Sora stood against him, and defeated him. Xehanort fell, but he took Riku along with him as his vessel. After turning into a Heartless to restore Kairi's heart, then regaining his shape from Kairi's own heart, Sora went out to save Riku from Xehanort and the rest of the Heartless in End of the World.

Xehanort used the Heartless to try and annihilate Sora so that he could unlock Kingdom Hearts, but he was bested once again. Xehanort proceeded to reaching out to Kingdom Hearts to try and control the darkness from within, however, Sora spoke out, saying that he was wrong, and in actuality, Kingdom Hearts was made of Light. The light shined through toward Xehanort, destroying his dark-fused body, giving Sora enough time to try and seal the door.

Kingdom Hearts: Chain of Memories
The Heartless appear inside Castle Oblivion as pawns of Organization XIII. It is unknown if the Heartless in the castle are crafted from Sora's memories or they are real Heartless. It is later suggested that even at this point, the Organization was manipulating Sora into collecting hearts for them.

Kingdom Hearts 358/2 Days
Roxas was sent by Xemnas, Xehanort's Nobody, to work with other Organization members and release the hearts of every Heartless that he encounters. Since only Emblem Heartless release hearts, the Organization only seeks to slay their kind; Pureblood Heartless are seen only as obstacles. This also explains when you destroy a Pureblood heartless, such as a shadow, on a 'collect hearts' mission, they do not add to your mission gauge.

However, on several missions, Roxas must destroy a number of Purebloods to complete the game. For example, during the game, Roxas must enter Wonderland to destroy a Novashadow.

Roxas's sole purpose in the Organization is to destroy Emblem Heartless, thus completing Kingdom Hearts and are the main enemy in the game.

Kingdom Hearts II
After Sora awakens in Twilight Town, he travels to the Mysterious Tower, where he meet's Pete, Maleficent's second in command, who is working to try and make an army of Heartless for Maleficent to use. After a skirmish with a couple of Heartless, Sora realizes that the worlds are still in danger. He meets with the Tower's master, Yen Sid, who explains Sora's new mission to him, though the Heartless have returned, they are not the main enemy this time, the Nobodies were. In almost every world, the Heartless appear alongside Pete, Maleficent, and one of the Disney villains. It was later revealed by Axel during a war on Hollow Bastion that Organization XIII was using Sora to release the hearts of every Heartless he destroyed so that the Organization could take them and create their own Kingdom Hearts.

After knowing this, Sora heads for the Organization's base, The World That Never Was, to try and destroy their Kingdom Hearts. After Ansem sacrificed himself to destroy Kingdom Hearts, the Heartless began moving together inside the Castle that Never Was, to seek out whatever they could to eat. Maleficent and Pete fought off the Heartless while Sora, Kairi, Riku, and the others went on to defeat Xemnas.

Gummi Heartless were introduced in this game. These Heartless often release items upon defeat while flying the Gummi Ship and defeating them. They are only fightable when flying the Gummi Ship in the passages to new worlds.

Kingdom Hearts coded
The Heartless appear once again as enemies inside a digital Journal.

Origin
According to legend, the people of the universe once lived in harmony on one world, with their hearts full of light. Unfortunately, they began to selfishly fight over the light, and darkness grew within their hearts, covering everything and destroying the world. The light survived in the hearts of children, who were able to rebuild the broken fragments of the one world into many, smaller worlds. However, the true light was still hidden in the darkness, so the worlds remained separate. As manifestations of that Darkness, natural Heartless have existed within the Realm of Darkness ever since. However, due to the walls between the worlds, they could initially only enter the Realm of Light when summoned.

About a decade before the events of Kingdom Hearts, Ansem the Wise, king of Radiant Garden, took it upon himself to study the darkness within human hearts, so he and his apprentices, Xehanort, Braig, Dilan, Even, Aeleus, and Ienzo, conducted experiments on the heart beneath his castle. While these tests began with simple psychological tests on willing subjects like Xehanort, they quickly snowballed and eventually caused their subjects' hearts to collapse, producing a Heartless. Unable at first to determine what they were, Xehanort named them "Heartless" due to their apparent lack of emotion. After finding that the Heartless were naturally drawn to the worlds' Keyholes, he invented a machine within the Heartless Manufactory that recreated the conditions which naturally spawned Heartless in order to create artificial Heartless. In order to differentiate the natural and artificial Heartless, Xehanort set the machine to mark its creations with the castle's emblem, and named the two types "Pureblood" and "Emblem", respectively.

After a visit from King Mickey Mouse of Disney Castle, Ansem officially ceased his experiments, but they continued to be performed by his apprentices, while he was exiled by them to the Realm of Nothingness.

To further their research into Heartless, as well as the Keyblade and the Princesses of Heart, the apprentices cast off their bodies and surrendered their hearts to darkness, giving birth to their Heartless and Nobodies. Against expectations, Ansem, Seeker of Darkness retained his human form, as did the apprentices' Nobodies. Eventually, the witch Maleficent learned of the Heartless and began using them in her quest for power, and gave the power to lead the Heartless to those who joined her in her quest.

Nine years before the events of Kingdom Hearts, she sent a swarm of Heartless to conquer the Radiant Garden, causing it to be lost to darkness so completely that only the castle that became known as Hollow Bastion remained, even in the survivor's memories.

During this time, whether from the machinations of Ansem's apprentices or Maleficent herself, the Heartless became able to invade the Realm of Light at their own choosing, and whole worlds began disappearing to the darkness.

Nature
Pureblood Heartless are natural Heartless, born from the darkness in people's hearts. 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, without a heart leaving it's body.

Emblem Heartless, on the other hand, were originally created from machines that reproduce the process of a heart being consumed by darkness. Afterward, stolen hearts can become Emblems. They have more varied shapes, colors, and are branded with Xehanort's symbol to differentiate them from the Pureblood ones. They are more common on the worlds within the Realm of Light, and usually mimic the shape of creatures or objects within that world.

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 act on instinct, their only goal to find hearts, and consume them to 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, 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 and 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 Invasion
While the Heartless were not researched with the intent of creating a fighting force, their very nature as manifestations of the destructive force of darkness made them naturally suited for conquering and destroying the various worlds. Their natural ability to travel to various worlds through the use of Corridors of Darkness makes it virtually impossible to fully defend a world from Heartless, and the method in which they are created and exist make it practically impossible to cut them off at the source. Even fighting them once they appear is made difficult due to their immunity to standard weaponry.

While sealing a world's Keyhole will save the world itself, it is still possible for the Heartless to appear on the world and hunt the people themselves. For example, even though the great Door to Darkness at The Radiant Garden was sealed by Sora in the Kingdom Hearts, the Heartless were able to remain on that world even after the destruction of the End of the World and the restoration of the worlds, and propagated to form a gigantic siege engine of over a thousand Heartless.

The Heartless's natural danger is only increased due to the many villains who seek to harness the creatures' power for their own goals. Many of these villains, such as Pete and Organization XIII, specifically seek to create abnormally strong Heartless by forcing beings with strong hearts to succumb to darkness.

The primary weapon for destroying the Heartless is the Keyblade, as it is both specially suited for manipulating the bonds between heart, body, and soul which create Heartless, and it is the only object able to thwart the Heartless in their goal of devouring a world's heart. However, in the absence of the Keyblade there are several other options. Magic is effective against the Heartless, as are magical weapons. In either situation, invading Heartless can be made disorganized by defeating the major Heartless which leads them, or the dark heart which attracted them to that world. While this does not by any means purge the world of its Heartless, it gives no small respite from their threat.

End of the World and Kingdom Hearts
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.

Types
Note- A name in bold denotes a boss.

Trivia

 * Ansem, Seeker of Darkness appears to have characteristics of both Emblem and Pureblood Heartless, in both his human and world forms. In both forms, he carries the Heartless emblem on his chest, though in human form it could possibly be an article of clothing. Notably, though, the similar outfit worn by Master Xehanort does not possess the emblem. Furthermore, the shadowy symbiote attached to him in both forms possesses the "hollow heart" shared by many other Pureblood Heartless. Though it is uncertain what the situation of this Heartless is, it is possible that this makes him both an Emblem and Pureblood Heartless.
 * In Kingdom Hearts 358/2 Days, a Tentaclaw consumes a Creepworm. This is the only known instance of Heartless eating another Heartless.