A heart (心 kokoro ) is one of the three parts which comprise a being in the Kingdom Hearts universe, the other two being body and soul. Hearts are one of the key points in the game's mythology, and the antagonists often seek to use them for their own gain.
What actually makes up a heart is a mystery in the Kingdom Hearts universe. Much research has been done to try and unravel the mystery, most notably by Ansem the Wise and his apprentices. However, this research only served to lower the barriers between worlds, set up the events for Kingdom Hearts, Kingdom Hearts: Chain of Memories, and Kingdom Hearts II, and cause many worlds to be plunged into darkness. It is known, however, that the heart is primarily made up of light and darkness, the latter of which formed due to people's greed. The exceptions to this are the Princesses of Heart, maidens whose hearts are free of darkness, and hearts artificially made devoid of light or darkness, such as Ventus and Vanitas.
The heart appears to be responsible for the composure of memories, will, goals, and ambitions, as is the case in the transfer of Master Xehanort's heart into Terra and the internal struggle of wills inside Terra-Xehanort. Of course, this is not to say that a being without a heart cannot have these things, as Nobodies inherit them from their original selves.
The heart is also responsible for emotions, as beings lacking one, such as the Nobodies, cannot feel emotions.
 A Tie-in to the Worlds
According to legend, the worlds were one and united through the light in people's hearts. Once these hearts became filled with darkness, however, chaos resulted, and the worlds were separated. It was the light that remained only in the hearts of children that would keep the worlds in existence before they were shrouded in darkness completely.
 Ansem's Research
Scholar Ansem the Wise was struck by a great curiosity of the heart and its true nature. He began research on the topic so he could better protect his world from the forces of darkness, together with his six apprentices, and started to write his reports. However when Ansem reliazed that his experiments could bring disaster to the world he stopped all his research.
However Xehanort decided to continue the experiments and realized that when hearts were seduced by darkness, a creature he dubbed a "Pureblood Heartless" was born. He made note of how Heartless acted on instinct, and wished to try creating his own Heartless. To tell them apart from the Purebloods, he dubbed them "Emblems" and christened them with what would later become the Heartless insignia. He made note of how they behaved relatively the same way, continuing to experiment in other areas with which the heart was concerned. He had no idea that he had caused chaos to brew. However when Ansem became aware of Xehanort's activities, he ordered that his research be stopped at once. This strict request would lead to the birth of Xehanort's Heartless and Xemnas, as well as the rebellion of Ansem's other apprentices. Xehanort and the other apprentices' Nobodies would later grow to form Organization XIII, a group that had the sole purpose of unlocking the secrets of the heart.
While Ansem was cast out into the Realm of Nothingness, Xehanort wrote the Ansem Reports. After he put his research and his findings to paper, these reports scattered across the worlds, and were able to help the residents of Traverse Town and Sora in his first journey when it came to learning about and fighting the Heartless. Ansem's experiments on the heart would continue to haunt him even after his escape from the Realm of Nothingness, throughout the story of Kingdom Hearts II, as he came across the reports and heartless spawned by the research of himself and his apprentices. He was forced to recall his past mistakes and even failed to succeed in avenging his dignity, realizing all of his experimentation had gotten him nowhere. Upon his supposed death, which instead erased the majority of his memory and sent him to the Dark Meridian, one of the last things he would hear was Xemnas's accusations that he was the source of all Heartless, and that the chaos and turmoil that had resulted over the past years were all his fault.
 The Seven of Pure Light
According to legend, the χ-blade shattered into 20 pieces, 7 of which were of light. These 7 pieces then became hearts of pure light. The bearers of these hearts are known as "Princesses of Heart," because they are always female, for reasons unknown. Because they are needed to access Kingdom Hearts, the Princesses of Heart were sought by beings of the darkness who wished to rule the Heart of all Worlds, particularly Maleficent and Xehanort's Heartless. Kingdom Hearts could only be accessed by the completion of a Keyhole in Hollow Bastion, which required all seven pure hearts to be joined as one in the Keyblade of Peoples' Hearts. These hearts were released, however, due to Sora keeping Kingdom Hearts out of the villains' reach.
 Powers and Abilities
A cherished element throughout the Kingdom Hearts series, hearts possess many unique powers and abilities. It is for this reason that antagonists seek them. Hearts are able to produce emotion, something that members of Organization XIII crave. Strong hearts are also able to feel the emotions of those connected with them, demonstrated many times by Sora, who has unconsciously felt the sadness of Roxas, Ventus, and Xion, as well as Vanitas, who was able to feel some of what Ventus felt. As shown with Kairi and Ventus, if one's body remains intact when the heart is lost, and is not turned into a Nobody, the person will enter a state similar to catatonia until the heart is restored. Ventus, in particular, has been in this state for over a decade, without a change in his health, despite there being no one to look after him. It is unclear if he has aged.
Hearts are also able to merge with others or be split, as seen when both Xehanort and Eraqus join hearts with Terra, and when Ventus's heart merges with Sora's. A much later instance of this would be Kairi's heart merging with Sora's during his first journey. They can be split to produce an embodiment of either the light or the darkness, as seen when Master Xehanort produces Vanitas by unlocking the darkness of Ventus's heart. Hearts can also be merged to create unique Keyblades, major examples being the χ-blade and the Keyblade of People's Hearts. These Keyblades ultimately grant access to the grandest of hearts: Kingdom Hearts.
The heart also has a heavy influence on all that lives, having the power to produce Heartless and potentially Nobodies at its loss or corruption by darkness. As the heart grants emotion, negative emotions formed when Vanitas and Ventus were split caused the Unversed to spawn, hence their connection to the heart.
Hearts removed from their bodies, but not transformed into heartless, lose some of the limitations of a physical form. In addition to being able to enter other peoples' bodies, they can move through time itself, as long as a version of themselves is there to greet them, as revealed in Kingdom Hearts Dream Drop Distance. In the original Kingdom Hearts, some disembodied hearts manage to escape the destruction of their worlds, becoming Summon Gems. Through the magic of the Fairy Godmother, Sora is able to use them as summons--temporarily giving them form, so that they may fight by his side. They do this through their own will and are fully conscious of themselves, as revealed when Simba and Mushu remember their adventures with Sora in Kingdom Hearts II. Finally, Ventus's and Terra's Keyblades, presumably directed by their hearts, rescued Aqua at one point in the Realm of Darkness, while Ventus's heart eventually resurfaced inside of Sora's to protect him from being corrupted by a Nightmare Dream Eater.
Magic is able to enhance the powers of a heart. In addition to the Fairy Godmother's ability to empower Summon Gems, Aqua created the Wayfinders, which allowed the user to temporarily gain power and abilities from the hearts connected with them. She also cast a similar spell on Kairi, which would lead her to someone who could protect her if she was ever in danger.
Newly born hearts have even stranger, more unique abilities. Sora's was able to seek out and unite with the broken fragments of Ventus's simply because Ventus was brought to Sora's homeworld. As a result of this, Vanitas took on an appearance similar to Sora's, and Sora's Nobody would later take on Roxas's appearance. The merge stabilized Ventus's heart, allowing him to eventually recover. Xemnas and Xigbar also revealed that a new heart could be "nurtured," even to the point where it became an exact replica of another person's heart, although they implied that they needed the power of Kingdom Hearts in order to do so.
 When Hearts are Broken
Naturally, when such an important object is damaged, the effects are catastrophic. The most obvious example is when the heart gives in to darkness, becoming a Heartless. Hearts either become trapped in monstrous shells, as in the case of Emblem Heartless, or physically shattered and rearranged to create those monsters, as with Pureblood Heartless. In both cases, the transformation is usually so extreme that the victim loses his or her sense of self, and if they are restored, they have no memory of their Heartless state.
Even after Sora was restored as a complete being, his corruption continued to plague him. The darkness in his heart would sometimes take over his Drive forms, causing him to transform into the feral anti-form. Although he showed no signs of the usual heartless behavior in this state, it was still a danger, because he couldn't heal or wield the Keyblade while in this form, and could not willingly change back. His Heartless remained alive inside of his memories, corrupting the journal that recorded his journey, growing exponentially more powerful, and attempting to reclaim his body.
A disembodied heart that is not turned into a heartless may sink into a semiconscious state, as with Kairi, who was unable to interact with the world, except to appear as visions to Sora.
When Master Xehanort divided Ventus's heart, Ventus was left catatonic. The last remnants of his heart fading away, he would have died, had Sora not combined hearts with him. Even after this, it took an unspecified but lengthy amount of time for Ventus to return to a sense of normalcy. During this time, if one attempted to interact with Ventus for more than just basic greetings or instructions, he would scream in pain and collapse. Ventus did not regain his memory of what happened to him until Master Xehanort explained it to him. In the novelization of Birth By Sleep, Vanitas also suffers extensively from feelings of incompleteness and inadequacy. After Ventus defeated Vanitas, Vanitas seemingly returned to Ventus's heart, his consciousness vanishing without a trace, while Ventus sank back into catatonia.
Even Master Xehanort suffered some ill effects from messing with his heart. He was not able to completely control Terra's body, and the aftermath of their clash of wills gave him amnesia. Ultimately, he had to give into darkness and become a Heartless in order to get his plans back on track. This heartless was incorporeal, being forced to manipulate events from behind the scenes, and could not use a Keyblade unless controlling the body of another Keyblade wielder. His Nobody inherited his amnesia, and was also unable to wield a Keyblade, for reasons not fully understood. It took Master Xehanort over a decade to regain himself fully.
 Kingdom Hearts
Kingdom Hearts is the largest and grandest heart of all. The heart of all worlds, it is a source of fantastic power and wisdom, and as such is sought out by many villains for different reasons. The artificial Kingdom Hearts of Kingdom Hearts II takes on the shape of a heart shaped moon, which further reinforces the fact it is the heart of all worlds. Upon a heart's release after the defeat of an Emblem Heartless, these hearts travel to Kingdom Hearts, empowering it, and in the case of the Kingdom Hearts II storyline, benefiting the Nobodies. However, when a Pureblood Heartless is defeated, no heart appears.
When a being, be they good or evil, gives in to the darkness in their heart, their heart is consumed by it and becomes a Heartless. This new entity separates from the body and soul, which fade into darkness, and begins to act on a primal instinct to gather other hearts and corrupt them. This instinct results in Heartless attacking other beings to increase their numbers, and attacking the heart of a world by means of the Keyhole, causing it to be completely overwhelmed by darkness. Of course, as dictated by their instinct, the Heartless seek out the biggest heart of all: Kingdom Hearts.
The amount of darkness in the heart, as well as the strength of the heart, seems to dictate the strength of the Heartless formed. As the Princesses of Heart contained no darkness in their hearts, they could not create Heartless. Likewise, Sora's heart contained so little darkness, his became a small Shadow, though his heart was immensely strong. This let him keep his emotions and memories instead of fading to the darkness, and though it could not speak, his heartless displayed no aggression and merely sought out his friends. Xehanort's heart was so strong and full of darkness that he was able to fully retain his sense of self, and his heartless took on his original form when not in his incorporeal state. This heartless, Ansem, Seeker of Darkness, has been implied to be the most powerful heartless currently in the Realm of Light, if not the most powerful heartless of all. The Heartless are arranged in a hierarchical society, with the most powerful Heartless directing the others' actions. This theory was proven by Xehanort whose Heartless went on to control all others.
Emblem Heartless defeated by the Keyblade give up the stolen heart, which causes it to return to the owner's original body. Heartless slain by another weapon return to darkness and eventually reconstitute elsewhere. However, in Kingdom Hearts II, the hearts went to Organization XIII, who captured the hearts within usually unseen dark corridors in the sky. Hearts will also not return to their owner if a Nobody was formed.
When a particularly strong heart is turned into a Heartless, the body it leaves behind begins to act of its own accord due to the strong will it possesses, becoming a Nobody. The body of a Nobody may be warped to a degree from its original form, depending on the strength of its heart. Members of Organization XIII all possessed remarkably strong hearts, allowing their Nobodies to retain their human appearance with only minor changes. Due to a lack of a heart, a Nobody cannot truly feel emotions; many members of Organization XIII often pretend that they can, following their memories of their human selves. The weaker Nobodies lose their sense of self, are made subservient to the Organization, and can even be modified beyond their original forms.
Although Nobodies are born as empty shells, they are capable of developing hearts of their own over time. Additionally, the shells can be implanted with the heart of another, becoming a vessel for that heart.
 Heart Collection
In Kingdom Hearts 358/2 Days, Organization XIII's heart collection process was greatly elaborated upon. While on several missions to various worlds, Nobodies of the Organization would seek out as many Emblem Heartless as possible, obtaining a captive heart from those they defeated. The captive hearts would then be absorbed by Kingdom Hearts, more specifically, Xemnas's synthetic incarnation of the world, immediately afterwards. Organization XIII completely ignored Pureblood Heartless, as they lacked the hearts they sought. The heart collection field was strictly where Xemnas found a use for Roxas, Sora's Nobody being the only one amongst their ranks able to actually release hearts, as he wielded the Keyblade. Any other Organization member simply caused a Heartless to fade back into darkness when it was defeated, without the released heart side-effect. Xion also possessed this power, as she was Sora, and to a lesser extent, Roxas's replica. While Xemnas planned on having Xion absorb Roxas so he could have the full power of Sora's Keyblade in the end, he desperately wished to reclaim Roxas after Xion was defeated and Roxas fled the Organization. He did not count on Roxas being captured by Riku in the Nobody's attempt to free Kingdom Hearts. This forced Xemnas to use Sora himself as an alternative.
In Kingdom Hearts II, Organization XIII switched tactics and used Sora for heart collection, because no surviving member was able to collect hearts after Roxas was absorbed by Sora. Sora remained unaware of his role in Xemnas's plan, sending captive hearts to Kingdom Hearts as he saved each world from the Heartless threat. When Saïx revealed this role to the Keyblade wielder, however, Sora became wary of defeating Heartless, as he did not wish to aid his enemies, the Organization. He had no choice, however, eventually reaching The World That Never Was, Organization XIII's stronghold. When King Mickey met up with Ansem the Wise, the elderly scholar attempted to encode Kingdom Hearts as data with a special device, the Kingdom Hearts Encoder. He realized, upon the machine's self-destruction, that hearts couldn't be encoded as data, destroying Xemnas's synthetic Kingdom Hearts and releasing every heart it held inside before not only his death, but the final battle in Kingdom Hearts itself between Sora, Riku and the Organization's Superior. The Nobodies were seen in the city below, waving their arms and presumably trying to locate their hearts, while the hearts that hit the darkness below the Castle That Never Was instantly transformed into Shadows.
It is unknown what became of hearts released in Kingdom Hearts. While Organization XIII did exist, they did not collect hearts until Roxas's birth, after the events of Hollow Bastion, and after he trained with the Organization members featured in Kingdom Hearts: Chain of Memories. It is most logical that these hearts went back to their bodies and returned to their original form.
Although the Emblem Heartless in Castle Oblivion release hearts, it is not clear whether or not these are real Heartless, or merely figments of Sora's and Riku's memories.
Unlike other enemies seen throughout the Kingdom Hearts series, the Unversed lack a direct connection to the heart. Instead, they are the embodiment of negative emotions, produced when Ventus's heart was split in half by Master Xehanort to create Vanitas. It is implied that they feed off of negative emotions let off from the hearts of others, growing stronger, and in turn make Vanitas stronger when they are slain and flow back into him.
To wield a Keyblade, one must have a strong heart, whether it be a heart of darkness or light, and they must have touched a Keyblade before, or inherited the ability from one who has. Roxas could wield a Keyblade even though he did not remember his time as Sora, but it took effort to get it to materialize in the beginning, and he was not aware of all of its properties, such as returning to the wielder's hand when called. Xemnas, however, did not wield a Keyblade, which remains a mystery. Nomura has implied that he intentionally wasn't using one, which some have taken to mean that Terra prevented it.
A type of Keyblade made from hearts also exists. The only known Keyblades of this type are the χ-blade, which is made from a heart of pure darkness and a heart of pure light, and the Keyblade of People's Hearts, which is made from the hearts of the seven Princesses of Heart. The χ-blade may also be forged from the clash of seven guardians of light and thirteen wielders of darkness, though this hasn't been elaborated upon yet.