Differences from Nobody, Heartless, and Unversed in Kingdom Heart series
Nobody
A Nobody (ノーバディ Nōbadi) is what is left behind by those whose hearts have been consumed by Heartless.
Nobodies are born when a Heart is swallowed by darkness and becomes a Heartless. They are the remaining parts left behind by the heart: the body, giving a Nobody form, and the soul, giving the Nobody life. However, only those with a strong will such as Sora are actually able to continue on as Nobodies. Those with the strongest wills manage to retain human forms (under special circumstances, sometimes the Nobody is formed with some slight changes to their appearance,) while the rest possess malformed, barely humanoid appearances; such examples as Nobodies like the Creeper and Dusk. Therefore, the more humanoid a Nobody, the more powerful it is. This is opposite the nature of the Heartless, whose form is more monstrous the stronger the original heart is (with the exception of Ansem, who retained human form because he gave into the darkness willingly).
All Nobodies vanish from the Realm of Light upon their creation at the loss of a heart and the creation of a Heartless. They are born in a world in-between the Realms of Light and Darkness, such as Twilight Town or Castle Oblivion. However, because of this and their lack of hearts, they are shunned by both Light and Darkness, and do not truly exist.
Because of their missing hearts, Nobodies are said to be incapable of feeling emotions by every credible source. Some humanoid Nobodies pretend to possess emotions, fooling others as well as themselves. On the other hand, Nobodies manage to keep their memories of their former lives, but unusual Nobodies lack memories of their former selves. Through these memories, Nobodies are able to remember what it was like to have emotions, thus allowing them to respond correctly to specific situations. Despite their lack of emotions, Nobodies are able to think for themselves and attack with definite planning (as opposed to the Heartless, which are mindless and function on pure animal instinct). Nobodies also seem to be able to experience physical sensations, such as pain, as they negatively react to taking damage, and Naminé passed out from the pain of being impaled on Marluxia's thorns (this was seen only in the Chain of Memories manga). The Kingdom Hearts: Chain of Memories manga and several other media forms provide an alternative view, saying that they do have emotions, but are plagued by a feeling of uncertainty and emptiness due to not having hearts. The manga further displays this "plagued" feeling by showing the members engaging in tasks like jigsaw puzzles, cooking, and reading to try to fill their emptiness.
Although never stated, the events of the Battle of 1000 Heartless implied that Nobodies are stronger than Heartless, since the Nobodies were able to easily overpower the Heartless. At the very least, they are just as powerful, and the fact that they are much more intelligent than Heartless only heightens the threat that they pose.
However, it seems that unusual Nobodies (like Roxas and Naminé) possess true emotions. Roxas and Naminé were born without any memories of their past lives due to the unusual nature of their births, and thus had no basis for any pretend emotions. Additionally, they co-existed with their original selves (Sora, the Keyblade Wielder of the Realm of Light and Kairi, a Princess of Heart, respectively) and held half of their essence within them, thus making them more human than other Nobodies. Although Axel denies the idea of feeling emotions, seems to show a bond toward Roxas and claims that Roxas makes him "almost feel like he has a heart" and has acted on his feelings of friendship a number of times.
Reference from :
http://kingdomhearts.wikia.com/wiki/Nobody
Heartless
The Heartless (ハートレス, Hātoresu) are beings of darkness that manifest in two forms, "Pureblood" and "Emblem". While most Heartless are in fact manifested hearts, they behave entirely devoid of emotion, and thus were named "Heartless".
According to legend, the people of the universe once lived in harmony on one world, with their hearts full of light. Unfortunately, they began to selfishly fight over the light, and darkness grew within their hearts, covering everything and destroying the world. The light survived in the hearts of children, who were able to rebuild the broken fragments of the one world into many, smaller worlds. However, the true light was still hidden in the darkness, so the worlds remained separate. As manifestations of that Darkness, natural Heartless have existed within the Realm of Darkness ever since. However, due to the walls between the worlds, they could initially only enter the Realm of Light when summoned.
About a decade before the events of Kingdom Hearts, Ansem the Wise, king of Radiant Garden, took it upon himself to study the darkness within human hearts, so he and his apprentices, Xehanort, Braig, Dilan, Even, Aeleus, and Ienzo, conducted experiments on the heart beneath his castle. While these tests began with simple psychological tests on willing subjects like Xehanort, they quickly snowballed and eventually caused their subjects' hearts to collapse, producing a Heartless. Unable at first to determine what they were, Xehanort named them "Heartless" due to their apparent lack of emotion.After finding that the Heartless were naturally drawn to the worlds' Keyholes, he invented a machine within the Heartless Manufactory that recreated the conditions which naturally spawned Heartless in order to create artificial Heartless. In order to differentiate the natural and artificial Heartless, Xehanort set the machine to mark its creations with the castle's emblem, and named the two types "Pureblood" and "Emblem", respectively.
After a visit from King Mickey Mouse of Disney Castle, Ansem officially ceased his experiments, but they continued to be performed by his apprentices, while he was exiled by them to the Realm of Nothingness.
To further their research into Heartless, as well as the Keyblade and the Princesses of Heart, the apprentices cast off their bodies and surrendered their hearts to darkness, giving birth to their Heartless and Nobodies. Against expectations, Ansem, Seeker of Darkness retained his human form , as did the apprentices' Nobodies. Eventually, the witch Maleficent learned of the Heartless and began using them in her quest for power, and gave the power to lead the Heartless to those who joined her in her quest.
Nine years before the events of Kingdom Hearts, she sent a swarm of Heartless to conquer the Radiant Garden, causing it to be lost to darkness so completely that only the castle that became known as Hollow Bastion remained, even in the survivor's memories.
During this time, whether from the machinations of Ansem's apprentices or Maleficent herself, the Heartless became able to invade the Realm of Light at their own choosing, and whole worlds began disappearing to the darkness.
Reference from :
http://kingdomhearts.wikia.com/wiki/Heartless
Unversed
The Unversed (アンヴァース Anvāsu), meaning "those who were not well-versed in their own existences", they are considered "the opposite of human life" and grow from the negative emotions produced when Vanitas was created from Ventus. It is because of this that Vanitas is able to control them. By all accounts, the Unversed are extensions of Vanitas himself.
Chronologically, the Unversed are either the first or second enemy referenced during Deep Dive, as they are the second race to manifest, but the first to attack the Realm of Light.
Reference from :
http://kingdomhearts.wikia.com/wiki/Unversed
References :
http://kingdomhearts.wikia.com/wiki/Nobody
http://kingdomhearts.wikia.com/wiki/Heartless
http://kingdomhearts.wikia.com/wiki/Unversed