- Those who are not human (人ならぬ者 Hitonaranu Mono )
- Overhunters (乱獲者 Rankakusha )
- Evil Spirit (悪霊 Akuryō ) - The term used by The Four Gods of Earth.
In the very distant past, Crimson Denizens, residing in "an adjacent world which cannot be reached", began to sense and resonate emotions given out by humans. Their desire to explore their curiosities as well as escape from the harsh environs of their world led to a Denizen to create a technique which allows them to cross the "Abyss between the Worlds" and arrive into our world. And by absorbing Power of Existence from humans, they would manifest in various forms and begin to roam our world in order to fulfill their various desires.
Prior to the "Spiral Organ" Leanan-sidhe's invention and popularization of Fūzetsu, most Denizens blended and mingled with human societies. In ancient times, they would openly appear to humans as various mythological gods, demons, fairies and even wizards. Though by the latter half of the 19th century, due to advances in human society as well as the increased use of Fuzetsu, they began to hide their true form and assumed human form.
Crimson Lords and Denizens have existed for millenia, and though they do distinguish between young and elderly Denizens, there is no mention of whether Denizens "die naturally" or even how old they are. Upon being "born" in the Crimson Realm, they immediately possess consciousness and knowledge of their own powers and begin their struggle for survival right away.
They possess emotions which are basically similar to that of humans, in which they are able to get drunk, enjoy food, feel angry at comrades getting annihilated, fall in love with each other or even with humans. As with humans, Denizens too have gender differences (though slightly different from humans in a stricter sense), and are fundamentally able to replicate their existences in a way which is similar to how living beings in this world reproduce.
Crimson Denizens which come to the human world usually possess two distinct names: the True Name (真名 mana ) and Alias (通称 Tsūshō ). True names are given to them since their birth in the Crimson Realm and refers to their traits and powers they possess. After coming to this world, Crimson Denizens usually named with the alias that mostly comes from 1) humans that fear and revere them 2) the utilization of other Denizen's aliases 3) mythical or folkloric names from human history or 4) one-self's fondness. In the human world, the true name becomes like a surname, while the alias functions as a first name.
Crimson Denizens, both allies and enemies, mostly call each others with their true names. In organization, some high ranking Denizens have their subordinates calling them only by their true names (e.g. Orgon and Vine). On the other hand, aliases, functioned like human' first name, are used with someone they are more intimated with (e.g. Flame Haze and their contracted Crimson Lords). However, when Crimson Denizens meet each other or enemies, they usually, as an etiquette, introduce themselves with both true names and aliases. As same as Japanese naming order, they introduce the true name (surname) first and then followed by the alias (first name).
Powerful Denizens are called Crimson Lords, though the difference between a Crimson Lord and a Crimson Denizen is only in their strength. For both Crimson Lords and Crimson Denizens, some would choose to manifest in human form, while others would opt to manifest in forms which closely match their "essence" from the Crimson Realm, thus appearing as "gods" and other mythological beings to humans.
While a Denizen's growth and survival depends on his/her innate talents, their ability to learn and adapt would also allow them to evolve into a Crimson Lord. An analogy is to compare a Denizen to a boat, and Power of Existence to fuels. A Crimson Lord is merely a larger ship which is able to contain more "fuel" than a normal Denizen. Though if both use up their fuels without refilling, they would both "sink" and be annihilated.
They are able to freely manipulate Power of Existence, as well as the ability to empathize with strong emotions and intentions from far away. There are many ways in which Power of Existence collected can be used, from manipulation spells to the creation of Rinnes to the creation of Treasure Tools.
The reason Denizens could absorb Power of Existence from humans is due to the similar spiritual and emotional foundations between the two species. While Denizens are also able to absorb the existences of non-humans, including dead objects, the Power of Existence provided would be insignificant for their needs, hence they mainly feed on humans. Though the forbidden technique, City Devourer, allows a Denizen to devour a huge amount of existences of both humans and non-humans by refining them into a highly concentrated existence.
Denizens are able to utilize the Power of Existence which they absorbed for various things, though the ability to control these existence varies between each Denizen. Crimson Lords are called as such due to their ability to manipulate large amounts of existences. Though for both Denizens and Lords, if the amount of existences used exceeded the amount of existence stored, then they would end up getting themselves destroyed.
- All Denizens choose to go by the name of a mythical or folkloric creature, deity, demon, or angel.