Soul

Soul (魂) refers to the spiritual being of Humans and Monsters.The Soul is the core of every entity, there is no "being" any deeper than it. By itself, it's extremely fragile and usually requires a three-fold Body to be able to operate normally, though some Skills might enable only those known as Otherworld Reincarnators to survive with just their Soul alone, resulting in their Soul strengthening and most likely acquiring Skills, sometimes including a Unique Skill.

Nucleic Heart
This is the core of the soul, where skills are engraved. It is not possible to remove a skill, though Greedy King Mammon can remove it under certain conditions.

Ego
This is the "being" of an individual, the memories and sense of self. An ego is the only part of the soul that could be removed from the makeup of a soul, which results in insanity. However, a being without an ego cannot access skills. It is described to be similar to a film of energy, wrapped around the nucleic heart. Reincarnators usually lose most of their ego, but sometimes strong souls, like Satoru Mikami, can retain their ego despite the dimensional travel.

Astral Body
This is the container for the soul and the form of a being. This layer requires a medium to preserve it, otherwise, it would slowly dissolve as the Magicules that make it up diffuse into the surrounding space and the ego and nucleic heart would be released, allowing for reincarnation. That medium is usually the body (physical flesh) but it can temporarily be the spiritual body. Those with the power to solidify the spiritual body are spiritual beings, like True Dragons, True Demon Lords, True Heroes, and Spirits, for some examples.

Traits and Names
Souls have different qualities; for example, only Humans can become True Heroes, and only Monsters can become True Demon Lords. Furthermore, Humans usually have stronger wills than most monsters and can control more skills, but Monsters' Souls can control more power. Thus, monsters are able to obtain Skills more easily and evolve into higher forms of their species. Monsters also start off as nameless with varying amounts of strength and intelligence depending on species and the environment.

Unnamed Monsters can be named and become Named Monsters, becoming the Namer's subordinate and causing a connection to form between the Namer and the Named. The Named Monster also gains strength based on the disparity in Strength between Namer and Named: the stronger the Namer, the stronger the Named Monster, though this process can also be adjusted according to the will of the Namer.

Soul Corridor
The connection formed between master and subordinate. For monsters, receiving a name is the quickest way to form one. Communication can be carried out via this link, and unlike Thought Communication, this link transcends time and space. The emotions of the speaker can be more keenly felt than with other methods of communication.

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