The Master of Hell is the primary antagonist of the anime Jigoku Shoujo/Hell Girl. The Master of Hell is a god or demon that possesses absolute control of the realm of hell, and intends to condemn as many people to hell as possible. It is not known whether he does this 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. In the mortal world and the in Realm of Eternal Twilight, the Master of Hell appears as a spider with eye-like marking on its back, however, in hell, he appears as an object in the sky consisting of four lines in an X-shape.
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 forces Ai to work for him 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.
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.
In combat, the only offensive ability the spider uses is a magic web of energy which he uses to bind his target, presumably to ferry their soul to hell. Ai, however, proves herself capable of resisting these bindings. The Master of Hell is also capable of teleporation and bringing up a shield to defend against energy attacks. Even without this shield, the Master of Hell proved himself completely unaffected to Yamawaro's magic attacks.
- 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 as many human souls as possible.
- 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.