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.
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.
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 strength of a heart seems to dictate the strength of the Heartless formed. The Heartless is arranged in a hierarchal society, with the most powerful Heartless directing the other's actions. Therefore, it makes sense that a strong heart will form a strong Heartless, which will in turn control other Heartless. This theory was proved 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 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 thus pretends to in order to achieve their ends, that being to finally be whole again by obtaining a heart.
Unbirth
Unbirths 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.
The Keyblade
Currently, the only known requirement to wield a Keyblade is to possess a strong heart, whether it be a heart of darkness or of light. This connection is expected to be further expanded on in the series.
