Justin Law, known as the Executioner, is a demon weapon who achieved the rank of Death Scythe, making him one of the most powerful weapons associated with the Death Weapon Meister Academy. He is historically significant as the youngest person ever to attain this rank, having accomplished the feat at the age of thirteen entirely on his own without a meister partner. His weapon form is a guillotine, a fact that heavily influences both his combat style and his epithet.

In terms of background, Justin is responsible for overseeing the DWMA's Western Europe Branch. His ability to function independently as a weapon is exceptionally rare, as most demon weapons require a meister to wield them effectively in battle. He gathered ninety-nine evil human souls and a witch's soul by himself to achieve his status, showcasing his prodigious talent from a very young age.

Justin Law is characterized by a devout and zealous personality, grounded in an absolute faith in Death. This faith is reflected in his priestly mannerisms, his way of dressing, and his habit of praying to amplify his soul wavelength. He is polite and speaks expressively to his allies, but he also possesses a serious and even mocking demeanor when facing enemies, such as the weapon Giriko. A defining quirk is his constant wearing of headphones, which play loud music. This results in him often being unable to hear what others say, leading him to communicate through expert lip-reading and to speak at a volume that others find excessively loud.

His primary motivation is rooted in his worship of Death and his loyalty to the DWMA. He follows orders to the letter and is committed to protecting the academy and its students. However, this devout exterior masks a deep internal struggle. Justin has difficulty forming friendships and trusting others, leading him to rely solely on his faith in a god. This solitary nature and his inability to form meaningful human connections make him vulnerable to madness.

This vulnerability defines his role and development in the story. Initially a trusted and powerful ally, Justin eventually succumbs to the madness wavelength emitted by the Kishin, Asura. After fusing with a Clown, a creature of madness, he undergoes a dramatic personality shift. His loyalty transfers from Death to the Kishin, and he becomes a traitor, joining Noah's Gang and later allying himself with the Clown Army. In this state, his personality becomes darker and more despicable, showing no remorse for killing his former comrades and even adopting a misogynistic viewpoint. His appearance also changes to reflect his new allegiance, with his priestly garments bearing the iconography of the Kishin instead of Death.

Key relationships in his life are often marked by distance or misunderstanding. He shares a rivalry with Giriko, another autonomous weapon. He also has a notable but one-sided relationship with fellow Death Scythe Tezca Tlipoca, who considers Justin a close friend despite Justin being unable to hear or understand him due to Tezca's mask blocking lip-reading. His fall to madness puts him in direct opposition to figures like Franken Stein and Marie Mjolnir, who recognize him as a significant threat.

Justin Law possesses a formidable array of abilities centered on his nature as a stationary-type weapon. He can perform partial transformations, manifesting guillotine blades from his arms to use in hand-to-hand combat without a meister. His techniques include the Holy Cross Chop, an attack where his arms become blade-like crosses, and Carcan Bras, which allows him to immobilize an opponent with part of a guillotine. His signature long-range attack is the Law-Abiding Silver Gun, a powerful ray of light made from his soul wavelength that can cleave through massive foes. By praying, he can amplify his soul wavelength to execute these techniques. After succumbing to madness, he gains the ability of Madness Fusion, merging with a Clown to gain enhanced durability, chain blades for hands, and the ability to propagate a madness wavelength.