In the mortal world and the in Realm of Eternal Twilight, the Master of Hell appears as a black spider with a red back. On his back he has green and purple markings and three yellow eyes. When appearing as Kikuri, he is a little girl with brown hair with a red flower in it. Kikuri wears a purple and blue kimono and has very big purple eyes. In hell, he appears as what seems to be his true form, a jellyfish-creature floating in the sky consisting of four shining lines in an X-shape, that can also open up as eyes.
It is unknown whether he sends people to hell for some greater goal, or simply for his own amusement, but it is clear that the Master of Hell does not care if his victims are actually guilty of any sort of crime. He is very manipulative and strict, not allowing anyone to disobey him. Obviously he is also quite sadistic and gets enjoyment out of peoples suffering.
The Master of Hell first met Enma Ai over 400 years ago, after she had used her psychic powers to rise from the dead and avenge her sacrifice, destroying the village with fire and killing all of the inhabitants. The Master of Hell appears before her and forces Ai to work for him. He does so by holding the souls of Ai's parents hostage, threatening to damn them to "wander in darkness forever" if she does not uphold her end of the bargain and deliver more souls to him. She begs him and states it wasn't her fault but he denounces her of everything and warns her not to betray him.
The Master of Hell also ordered Ai to shut out all emotion, and threatens to condemn Ai herself to hell if she does not obey his orders, and often sends her into situations seeming in an attempt to elicit emotion. He then watched over her for hundreds of years, making sure she does her job correctly. Her companions are also extra careful when they notice him around. He tries to create missions for her, that make her reminiscent about the past and hopes she will break the rules, so he can punish her. When Ai let her emotions resurface and acted through hatred, the Master of Hell summoned her back to hell and stated, she would be finally sent there, due to her breaking the rules. Ai uses her power and is able to teleport away. He is angered by this, but decides to keep watching her, since she continues doing her job as Hell Girl.
He possesses a little girl as a vessel and meets Ai and her companions. They realize she is also a demon. They don't know his true identity, but take him, as Kikuri, with them. Kikuri then accompanies them on their missions. She acts very spoiled and rude, listening to noone except Ai. She often wanders off and messes around, causing even more trouble and often being responsible for a person sending someone to hell. He enjoys annoying all of the others. Eventually, he starts his main goal and infects an entire town with hatred. All the people start sending each other to hell, much to his enlightenment. When Ai refuses to send another innocent boy to hell, he decides to turn her into a mortal again as punishment. This causes her body to age 400 years in an instant and she is close to death. He then reveals that he was Kikuri the entire time to the others. He attacks them and tries to keep them from helping Ai, but was unable to. She sacrifices herself for the boy and wanders on as a spirit, going to neither heaven nor hell. The Spider decides not to punish her parents, as he was genuiently moved by Ai's sacrifice and thanked her for all the hard work.He later senses that Ai is returning and manifests Kikuri inside of a wind-up toy. He gathers all of her companions and then continues to accompany them like the previous time. The Master of Hell also fated Yuzuki Mikage to be the next Hell Girl, since after she died, her strong spirit managed to make her live on. He observes them and eventually transforms back into the spider and turned Yuzuki into the new Hell Girl. When she tried to abused her powers and send someone she hated to hell, he saw it as an immediate sign of disobedience and planned to send her to hell, because he once again states that the Hell Girl doesn't need a heart. However, Ai came back and sacrificed herself to become Hell Girl for eternity, allowing Yuzuki to go to heaven. The Master of Hell agreed to that.
When Michiru Sagae burns down her village and becomes a hateful spirit, he decides for her to replace Ai as the next Hell Girl. He brings her to the others and turns her into Hell Girl. Michiru does not want to send innocent people to hell, however and denies these requests. It's unknown how he reacted to her behaviour.
- The Master of Hell shares similarities to the Judeo-Christian conception of Satan (commonly known as The Devil), most notably his mastery of manipulation, dominion over Hell, and his desire to collect and condemn as many human souls as possible to Hell, regardless of whether they're guilty or not .
- It is suggested at the end of one episode that the Master of Hell may be a pawn of a more powerful being, when Wanyuudou describes a God that will condemn humans to hell for the slightest misgivings. It is not clear, however, whether Wanyuudou is describing The Master of Hell, or another, more powerful being.