Heartless

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

Kingdom Hearts Birth by Sleep
In an attempt to draw out the darkness in the heart of his pupil, Ventus, Master Xehanort summoned several Neoshadows to fight him, but Ventus was overwhellmed and nearly destroyed by the Heartless instead.

Much later, as Aqua wanders the Realm of Darkness following her battle with Terra-Xehanort, she encounters a group of Darksides. Although she initially summons Master Keeper, she decides to let the Darksides come at her without resisting. However, Terra's Keyblade and Ventus's Keyblade come to her rescue, a last symbol of hope from her old friends Terra and Ventus, and destroy the Darksides. Her spirit returning, and in hopes of returning to her home, Aqua presses on through the Realm of Darkness, along the way encountering many Pureblood Heartless, including Shadows, Neoshadows and Darkballs. During this time, she is stalked and attacked by a massive Heartless, engaging it in combat and emerging victorious after a strenuous battle.

Between Kingdom Hearts Birth by Sleep and Kingdom Hearts
Building on Ansem the Wise's research into the nature of the heart, two of his apprentices, Xehanort and Master Xehanort's co-conspirator, Braig, experimented on people's hearts without his knowledge, convincing Ansem's other apprentices, Dilan, Even, Aeleus, and Ienzo, to assist them.

During the experiments, Xehanort discovered the Heartless, and one of them leads him to the heart of Radiant Garden. After further studying them, he created the first Emblem Heartless.

After discovering his apprentices' experiments and realizing his error in trying to interfere with the nature of the heart, Ansem the Wise ordered his apprentices to cease their research. However, they overthrow Ansem, banishing him to the Realm of Darkness while Xehanort takes his name. Their continued experiements eventually lead the group to lose their hearts, and Xehanort's Heartless, Ansem, Seeker of Darkness, began planning to find and take the power of Kingdom Hearts.

In the meantime, the Heartless consumed most of the world, leaving only Hollow Bastion, and they began speading to other worlds. Ansem began manipulating Maleficent and her allies to use the Heartless in their bid to conquer all worlds and take Kingdom Hearts.

Kingdom Hearts
Maleficent sends the Heartless to invade many worlds, to consume their hearts and search for the seven Princesses of Heart. Many of her allies use the Heartless to advance their own plans, in many cases to conquer their own worlds.

Sora encounters Heartless within his Dive to the Heart, including a Darkside. A few days later, when Riku opens the door to the heart of the world, the Heartless invade en masse, and they succeed in devouring the islands, despite Sora obtaining the Keyblade and attemping to stop them.

After he escapes to Traverse Town, Sora meets Leon and Yuffie, who explain to him the nature of the Heartless and tell him his Keyblade both draws them to him and is the best weapon against them. This leads Sora to fight the Heartless alongside Donald Duck and Goofy, in an effort to save the worlds.

Sora eventually learns that the Heartless are being used by Maleficent to capture the seven Princesses of Heart and open the door to Kingdom Hearts, so that she and her allies can rule over all worlds. However, Ansem, possessing Riku's body, uses the Keyblade of heart to drown Maleficent in her own darkness and transform her into her dragon form. After she is destroyed by Sora, Ansem reveals that, like the other villains, she was being used by the Heartless.

Ansem challenges Sora in order to claim Kairi's heart and complete the Final Keyhole, but Sora defeats him. Sora then uses Ansem's Keyblade on himself, in order to free Kairi's heart and complete the Keyhole so it can be sealed. Although he succeeds, he loses his own heart and becomes a Shadow Heartless in the process. Kairi's light restores Sora to his human form, and he soon travels to the End of the World to stop Ansem from reaching Kingdom Hearts.

Ansem uses a great many Heartless in an effort to defeat Sora, but he is finally beaten in front of the Door to Darkness. Ansem reaches out to Kingdom Hearts to try and control the darkness he expects within, but he is destroyed by the light of Kingdom Hearts.

In order to stop an immeasurable wave of Heartless from emerging through the Door to Darkness, Sora closes and locks the door with the help of Riku and Mickey, who are stuck in the Realm of Darkness.

Kingdom Hearts: Chain of Memories
The Heartless appear inside Castle Oblivion as pawns of Organization XIII. Although it is unclear whether the Heartless in the castle are crafted from Sora and Riku's memories or are real, it is suggested that even at this point, the Organization is manipulating Sora into collecting hearts for them.

Kingdom Hearts 358/2 Days
As a Keyblade-wielding member of Organization XIII, Roxas is often sent on missions to defeat Heartless and collect hearts. During one of his early missions, Marluxia introduces Roxas to fighting Heartless and explains to him the difference between Pureblood and Emblem Heartless. He also tells Roxas that hearts he collects will gather to form Kingdom Hearts, and Zexion further emphasizes the importance of Roxas's role.

Kingdom Hearts II
After Sora awakens in Twilight Town, he travels to the Mysterious Tower, where he meets Pete, Maleficent's second in command, who is working to make an army of Heartless for Maleficent's use. After defeating his squad 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, telling him that although Sora and company prevented a catastrophic invasion of the Realm of Light by Heartless, there are still many Heartless throughout the worlds. Maleficent and Pete continue to use Heartless in attempting to conquer worlds, and as Axel reveals during the Battle of the 1000 Heartless, Organization XIII is using Sora to release the hearts of every Heartless he destroys to create their own Kingdom Hearts.

With this knowledge, Sora heads for the Organization's base world, The World That Never Was, to stop their plans for Kingdom Hearts. Inside the Castle that Never Was, Saïx shows off the Organization's Kingdom Hearts to Sora's group, and he summons a great many Shadows to attack.

Kingdom Hearts is damaged when Ansem the Wise fails to encode it as data, causing hearts to rain down over the Dark City, spawning a great swarm of Shadows that attack the castle. Maleficent of Pete take on the swarm, sending Sora and company to defeat Xemnas, atop the castle.

Kingdom Hearts coded
Digital representations of the Heartless appear within the datascape of Jiminy's Journal, just as they had appeared in  the real worlds. The bugs infecting the Journal also appear as various species of Heartless. The source of the bugs, the digital representation of Sora's Heartless, absorbs the defeated Heartless in order to increase in power, but it is defeated by Data-Sora and King Mickey, and the journal is repaired.

Kingdom Hearts 3D: Dream Drop Distance
Before beginning Sora and Riku's Mark of Mastery exam, Yen Sid explains that  the Dream Eaters are manifestations  of darkness that serve as the Sleeping Worlds' counterpart to the Heartless.

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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.

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 its 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 appearances become 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.

Special Heartless

 * Note: 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 otherwise identical 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.