Nobodies

Nobodies (ノーバディ, Nōbadi) is what remains of those who have lost their hearts to Darkness. The first generic Nobodies are seen; although Xemnas appeared as early, it was unknown that he was in fact a Nobody, when Master Jolnir shows them to Sora, along with the left members of Organization XIII. They are made of body and soul. Depending how strong their human hearts were, the body and soul will take a human or monster-like form.

All Nobody types are named after job classes in the Final Fantasy video game series, except for Dusks, Creepers, Twilight Thorn and several fought with the Gummi Ship.

The Nobody emblem resembles a splintered, upside-down heart. It originates from Terra's memories of the Terra's Mark symbol, which are inherited by Xemnas. It is also present on Master Xehanort's Keyblade Armor. The symbol first appears, where it marks the names of sleights used by Organization members and the Riku Replica, as well as the head of the Specter.

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 * —About the Nobodies.

Nature
Nobodies are born when someone loses their heart. 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 produce Nobodies. If a person's heart wasn't strong enough, then their body and soul will simply fade away. Those with the strongest wills manage to retain human forms, with only slight changes in appearance, while the rest possess malformed, barely humanoid appearances, such as with Creepers and Dusks. Nobodies become more humanoid depending on how strong their human heart was.

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 that were born from special circumstances, such as Roxas, 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 manga). The 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 the 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.

Nobodies do have the ability to regrow hearts, but the specific conditions to do so are currently unknown. Nobodies such as Roxas and Naminé seem to posses true emotions since they 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, he shares a bond with 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.

When defeated, Nobodies fade into darkness; however, when a person's Heartless and Nobody both are defeated, that person is restored to their original, complete form, with their memories from their time as a Nobody intact. Lea, Ienzo, Dilan, and Aeleus all return to their human forms after their Nobodies are defeated; presumably their Heartless had been defeated at an earlier point.

Though, it is revealed that Nobodies can form hearts of their own as they become more self aware, and that Xehanort had been lying to the rest of the organization about this fact all along.

Replica
A Replica (レプリカ, Repurika) is an artificial copy of another being that can be considered a special type of Nobody. The first Replica that appears in the series is the Riku Replica. Xion is later revealed to be another Replica, Experiment No. i in the Organization's Replica Program. In the manga adaptation of the Chain of Memories, Vexen also creates a large number of Replicas of himself, who survive into the events of Kingdom Keyblade Masters II.

While the exact method of creating a Replica is unknown, a couple different methods have been demonstrated for the copying process, with different results. The Riku Replica was created using data collected by Vexen during his fight with the real Riku. Xion, an imperfect Replica of Roxas, was created by Xemnas from Sora's memories. Due to her imperfections, Xion's appearance varied in the eyes of different people, but because of Sora's powerful memories of Kairi, she resembled her to those that were closest to her.

Despite their nature as copies, Replicas have demonstrated the ability to develop personalities and a powerful sense of self, and can also develop hearts, just like regular Nobodies. This creates a great deal of internal conflict when they discover what they truly are. When the Riku Replica discovers he is a copy of the real Riku, he becomes obsessed with proving himself to be more than a "fake," as well as for his existence to be his own. Xion, upon discovering what she truly is, starts referring to herself as a "puppet"; she is unsure of how to handle the revelation. In the manga, the Vexen Replicas are very quiet and sad compared to the original Vexen, telling the Riku Replica that they "are all failed replicas."

Replicas have unique abilities. They are capable of absorbing other individuals to increase their strength;the Riku Replica absorbs Zexion's life-force to steal the Nobody's power, while Xion exists solely to absorb Roxas and become "the new Sora," thus preventing the real Sora from ever awakening from his slumber.

Replicas, like Nobodies, fade into Darkness after they are defeated. Xion, however, is absorbed by Roxas, therefore she continues to live on inside him, as well as inside Sora.