There are three forms of being within the game world: living beings, heartless and nobodies. Here is the logic as explained by Yen Sid (the knowing wizard and diegetic tutor).
(Nobody and Heartless)
Sora: But wait a sec, how come the Heartless are still running around?
Yen Sid: Your past endeavors did prevent an immense effusion of Heartless from the great darkness. Make no mistake about that. However, the Heartless are darkness made real and darkness yet lingers in every heart. The Heartless are fewer, but while darkness exists in a single heart it will be difficult to eliminate them.
Goofy: Gorsh, that must mean if everybody's heart was full of light them Heartless would go away!
Yen Sid: Now it is time to speak of the enemies you will encounter. If one such as you, Donald, yields to the darkness in their heart they too will become a Heartless, but you know this. The Heartless are always lurking and ever seeking to capture new hearts. Never let your guard down. Now then, at times if someone with a strong heart and will, be they evil or good becomes a Heartless the empty shell they leave behind begins to act with a will of its own. An empty vessel whose heart has been stolen away. A spirit that goes on even as its body fades from existence. For you see, Nobodies do not truly exist at all. Nobodies may seem to have feelings but this is a ruse they only pretend to have hearts. you must not be deceived.
Sora: Nobodies. They don't exist
Yen Sid: Now then, the being you see before you is known as a Dusk. they are the most common form of nobody. but there are others. Some larger. Some with frightening and unique powers. be vigilant. on your journey you will meet an alarming number of Dusks. They will all attempt to do you harm. Still they are nothing but empty shells destined to return to darkness. But...the beings you see before you now are different. these powerful Nobodies have formed a group called Organization XIII. It commands the lesser Nobodies.
Sora: Organization XIII...
Yen Sid: While Heartless act on instinct, Nobodies function in a higher manner, they can think and plan and it seems they are working towards a goal. What that goal is we do not know. The king sensed the danger and journeyed forth to find it. He found the dark realms keyblade and closed the door. Now he's traveling from world to world fighting the heartless as he seeks the answer to the riddle of the Nobodies and Organization XIII.
Yen Sid: そなたたちの働きによって大いなる闇から大量のハートレスがあふれだす事態はさけることができた。しかし、ハートレスは元々世界に存在している心の闇が具現化したもの。数は減ったが、心に闇がある限り完全に消し去ることはむずかしい。
Yen Sid: では、おまえたちが戦うであろう敵について話しておこう。もし、ドナルドが自らの心の闇にやぶれてしまうと心無き者、ハートレスになってしまう。これはよいかな？ハートレスは相変わらず心を求めてうろついているから気を抜いてはいかん。さてー良くも悪くも、強い心を持った者強い思いを持った者がハートレスになると、残された抜け殻が意志を持ち動き出すことがある。これがノーバディだ。心を奪われし器ー魂の存在ー消え行く肉体ーいや、存在しない者。感情的に動いているように見えるが、実際は心があるかのように振る舞っているだけだ。奴らの言動に惑わされてはいかんぞ。
Yen SId: さて、このタイプのノーバディはダスクと呼ばれている。ノーバディの一般的な姿だな。これ以外にも大きいものや特別な能力を持ったノーバディがいるから注意が必要だぞ。よいか？通常のノーバディどもは数が多いので厄介だが、しょせんはただの抜け殻。悪さはするだろうがほどなく闇に溶けてしまう。しかしー彼らはノーバディの中でも特別な存在だ。ⅩⅢ機関と呼ばれる組織を作り他のノーバディを統率している。
Yen Sid: 本能的に活動するハートレスとは違い。ノーバディは統率され、何事かをなさんとしているようだ。しかし、その目的は見当がつかん。王は世界の危機にいち早く気づいて行動をおこした。闇の世界のキーブレードを見つけ出し扉を閉じた。そしてなお、ハートレスと戦いながらⅩⅢ機関ノーバディの目的を探そうと世界を駆けめぐっているのだ。
A being is thus comprised of various things, but mainly a heart, a body and memory. The heart is a repository for the various emotions (seen in the Halloween Town experiment as well as the Nobodies' lack of emotions) as well as somehow tied to both light and dark (known through the discussions between Xehanort's Heartless and Sora throughout the game). The body is a shell and both mistreated by the gameworld's logic and the characters: Pinocchio has a heart but is not really a body, and Nobodies are, in fact, a body even though they supposedly are an empty, non-existign shell (存在しない者). Finally, memories link/chain between people and in this way are one of the more important aspects of being.
As Yen Sid states, when a being gives in to the darkness in their heart they turn into a Heartless. In actuality, their heart is swallowed by the Heartless.
(Creation of a Heartless)
The Heartless is thus mindless darkness that has taken a heart. When the Heartless is killed the heart is then released into the world where it goes to Kingdom Hearts.
(A Heartless is killed and its heart goes to Kingdom Hearts)
The only instances of this not happening are the 7 princesses of Heart and Sora, all of which remain unexplained other than to give both Sora and Kairi Nobodies (Roxas and Namine respectively), but no Heartless.
The Heartless seek only hearts and as sucha re depicted as rather mindless creatures. Base and animalistic - primal evil, rather than 'cultured' evil.
On the other side of the equation is the Nobody that is formed when the being is strong. The Nobody is the body, the empty shell. It has no heart as the heart went to the Heartless, and it has no memories of who it was. It has memories of the present, but cannot form emotions. As such, the heart is posited as intrinsically tied with memory and what might be called the essence of being is somehow tied with both but unexplicated. When Sora is turned into a Heartless/Nobody he claims 'he' was floating in teh darkness, but that his ties to Kairi brought him back. His soul/essence is tied to memory and others.
If the Heartless are base, animalistic creatures the Nobodies are posited as strategic evil.
In a franchise so primarily about transnationalism and territory the question now becomes how does one read these forms of being? The heroes of liberal agency that has been given an equal pattering of the social subject, but what of the rest? At this point I'm not sure.