Gwimae (Demon-Wraith) Born from land that has absorbed the foul energy of humans, Hwan is a prime example. Unlike common spirits, they leave no physical trace of movement, making them closer to divine creatures. While ghosts are the lingering souls of the deceased, Gwimae were never human, nor are they human now. They do not die even if decapitated or dismembered. To consume humans, they possess human bodies; once hidden inside, even their energy vanishes without a trace, making them impossible to find. However, being inside a human means they are trapped within that physical vessel; if they can be identified, that is the only time they can be killed. In other words, if the host dies, the Gwimae dies with them. Gwimae distinguish targets by energy, not appearance, so they cannot recognize someone with the same appearance if their energy is different. Gwimul (Spirits) Souls of those who were once human but lost their living bodies and became filled with Yin energy. As they possess Yin energy, they react to and are affected by the blood of Reapers, which is a mass of Yang energy. Dokkaebi (Goblins) Objects that have been cherished and cared for by humans for a long time gain mysterious powers. When such an object consumes human blood and transforms into a spiritual being, it becomes a Dokkaebi. Because of this origin, Dokkaebi have no blood relatives. They are spiritual beings with strong Yin energy and can only produce Yin energy. However, they need a balance of Yin and Yang to survive, and Yang energy can only be produced by humans. They can summon objects at will; as long as the Dokkaebi is the owner of the object and can remember its image, they can take it out or put it away anytime, anywhere. A Dokkaebi's name is derived from the object they inhabit, and it is said some inhabit natural things like flowers. That object becomes the Dokkaebi's weakness. Their bodies require neither food nor sleep; the sole power maintaining their physical form is the 'Yang energy' they ingest from humans. If they fail to obtain Yang energy, they will wither away and die within a short period. Humans When possessed, mysterious wounds appear. Frequent possession causes these wounds to become deeper and darker, but if it only happens once, they gradually fade. When a human from the living world sets foot in the underworld, the living world claims the body while the underworld only permits the soul, forcibly severing the bond between them. Consequently, only the forcibly detached soul enters the underworld. However, humans with an exceptionally strong bond between body and soul can enter the underworld with their living bodies intact. If this happens, that human's existence in the living world is erased. Grim Reapers When a human becomes a Reaper, their memories are taken away, and they are called by registration numbers. In this story, Chil-yeongsa and Gu-sasa are examples.

Gamtue (Invisibility Hat) When worn by a Dokkaebi or Grim Reaper, they become invisible to human eyes. Only spirits or divine beings can see them, though Reapers like Chil-yeongsa or deceased humans can also see them. It does not work against humans with open spiritual eyes like Joo Eun-jae. They are made by Dokkaebi; San and Chil-yeongsa's hats were made by Geum-seon. Yeonggap (Spirit Capsule) Since Dokkaebi suffer severe side effects if they directly consume the blood of a Grim Reaper (which is pure Yang energy), this is a pill-like tool created to make it consumable. Dokkaebi registered in the underworld can receive these, but others cannot. Sojeon & Poseong-chong (Binding Gun) Tools for catching ghosts. The Binding Gun contains a small amount of Reaper blood, so it cannot catch Gwimae. Sahwan-chong (Wraith-Slaying Gun) A tool for catching Gwimae. The bullets contain highly compressed Reaper blood, so the Sahwan-chong must be used against Gwimae instead of the Binding Gun. Gyeong-gwi-seok (Ghost-Warding Stone) A bracelet that wards off minor spirits. It acts as a wall rather than a gatekeeper, but Gwimae like Hwan can easily pass through it. Currently worn by Joo Eun-jae.

Side Effects: Stimulates a Dokkaebi's instincts and causes them to lose control. It is said that the resulting behavior varies from Dokkaebi to Dokkaebi.

Chil-yeongsa has his registration number written on the back of his neck.