Heart

A heart 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.

Construction
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 exception to this are the Princesses of Heart, maidens whose hearts are free of darkness. It is unknown if males can also possess this quality.

The heart is also responsible for emotions, as beings lacking one like the Nobodies cannot feel emotions.

A Tie-in to the Worlds
According to legend, the worlds were one and united through the light in peoples' 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, realizing 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 now idea that he had caused chaos to brew. When it dawned upon him that his research had brought ruin and destruction to the worlds, he demanded the experiments on the heart and the Heartless were 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 soul purpose of unlocking the secrets of the heart.

While Ansem was cast out into the Realm of Nothingness, he wrote the Ansem Reports to hang onto sanity. He put his research and his findings to paper, these reports scattered across the worlds, and able to aid the residents of Traverse Town and Sora in his first journey when it came to learning about and tackling 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 his reports continued to spawn as the Heartless he had created returned. He was forced to recall his past countless times and even failed to succeed in avenging his dignity, realizing all his experimentations had gotten him nowhere. Upon his death, one of the last things he would hear would be Xemnas's accuasations 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
Supposedly, seven maidens existed with hearts of pure light, whose hearts when joined as one could unlock Kingdom Hearts. It was for this reason that the maidens, called 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.

It's worth noting that unlike other hearts(which are heart-shaped), a heart of pure light looks more like a glowing sparkle.

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, benefitting the Nobodies.

Heartless
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 couldn't create Heartless, because their hearts contained no darkness. 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.) The Heartless is 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.

Defeated Heartless give up the stolen heart, which causes it to return to the owner's original body. However, in Kingdom Hearts II, the hearts went to Organization XIII, which may mean there is a way to change the flow of hearts. Hearts will also not return to their owner if a Nobody was formed.

Nobodies
When a particularly strong heart is turned into a Heartless, its body begins to act of its own accord due to the strong will it possesses. This being is called a Nobody, a being which does not truly exist. Depending on the strength of the heart, the body may or may not be warped. Members of Organization XIII all possessed remarkably strong hearts, causing their Nobodies to retain a human appearance. Due to a lack of a heart, however, a Nobody cannot truly feel emotions and in Organization XIII's case they pretend to in order to achieve their ends, that being to finally be whole again by obtaining a heart.

Heart Collection
In Kingdom Hearts 358/2 Days, Organization XIII's heart collection process was greatly elaborated. 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 when Sora's Nobody fled the Organization. He did not count on Roxas being captured by Riku in the Nobody's attempt to free Kingdom Hearts so he could revitalize Xion, Xemnas forced to use Sora himself as an alternative.

In Kingdom Hearts II, Organization XIII switched tactics and used Sora for heart collection, no surviving member 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 master, 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 Castle 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 and the Organization's Superior.

It is unknown what became of hearts released in the first 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 Chain of Memories. It is most logical that these hearts were absorbed by the true Kingdom Hearts and were never to be seen again.

While hearts are released when Sora defeats Emblem Heartless in Kingdom Hearts: Chain of Memories whilst he treks through Castle Oblivion, these Heartless were of his memories just as the worlds he encountered them were, thus the hearts they held were merely figments. Hearts released in the 13th Floor of Castle Oblivion, a true "world" even though it was accessed via a card made of Sora's memory, were most likely absorbed for this exact reason, the fact that they were the only true hearts in the entire castle.

Unversed
The Unversed are an enemy new to the series, and it is unknown what connection they have to the heart, and if they even do have one. However, Tetsuya Nomura has stated they have a connection to the Heartless and Nobodies, implying that they too will be connected to hearts. It is expected that this connection will be revealed in the upcoming game, Kingdom Hearts Birth by Sleep.

Keyblade
Currently, the only known requirements to wield a Keyblade are having a memory of wielding a Keyblade and possessing a strong heart, whether it be a heart of darkness or light. This connection is expected to be further expanded on in the series.