"But then we came to the Heart, the most complicated bloody thing in this goddamn universe. We had no clue what it was, or even what is did; but we knew it existed in some form, because the Heart was what allowed for the universe, and by extension Prana, to recognize a being. What was the Heart? We had to know." -Cynthia, Former Queen of the Athenian Empire
A Heart is one of the three theorized parts which comprise a being in the universe, the other two being Body and Soul. Hearts are one of the key points in Requiem's battle strategy, often using and manipulating them in various ways to strengthen their armies. What actually makes up a heart is an utter mystery in the universe, as the Heart is an extremely complicated and often undetectable subject. Many have taken the obvious route of suggesting that it is an ethereal concept which resides inside the body, but more scientific minds have theorized that it is an aspect of consciousness somehow recognised by the universe itself similar to Thought in telepaths or the Warp. Much research has been done to try and unravel the mystery, most notably by the ancient Ventus and the Athenian Empire. However, this research only served to either descend scholars and scientists into madness or cause the destruction of entire societies from rampant experimentation into Darkness.
It is known, however, that the heart has an extreme connection to light and darkness; which allows for Alignments to be formed by the general outlook and histories of nations in the Hearts of their people. But it does not seemingly determine the personality or morality of a person in any noticeable way, so it is often debated whether Hearts hold any purpose at all. Hearts can be found in nearly all sentient species, including androids and clones, as they have little to no connection with actual biological hearts; which mainly serve as vessels for the Heart. It is often theorized that emotion is regulated by the Heart, and that any hypothetical being lacking one would be devoid of emotion. And it is postulated that Hearts allow for Mana to be manipulated by Mana-capable races, allowing for magical feats to be accomplished; but these theories have yet to be tested or confirmed.
Hearts can collapse when subjected to extreme amounts of Darkness, forming Heartless from their broken remains out of the Darkness which surrounds them. This new entity separates from the Body and Soul, which fades away (at times becoming Nobodies, 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, along with creating Cornerstones out of the Darkness accumulating in a planet to propagate Negative Energy Fields. Those with dark and powerful Hearts, despite not being Heartless, are able to control them provided they possess a Keyblade; and are able to pass on their control to similarly dark beings.