Heart

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.

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.

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.

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 Ansem 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 his reports continued to spawn as the Heartless he had created returned. He was forced to recall his past mistakes and even failed to succeed in avenging his dignity, realizing all his experimentations 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 would be 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
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.

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. As revealed in Kingdom Hearts Birth by Sleep, hearts are also able to sense the emotions of others from outside worlds if they are strong, as seen when Sora feels Ventus's great sadness after the loss of his heart. As shown with Kairi and Ventus, if one's body remains intact when the heart is lost, the person will enter a state similar to catatonia until the heart is restored. Hearts are also able to merge with others or be split, as seen when both Master Xehanort and Master 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 X-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.