The character known as Violence Jack emerges from the rubble of the Great Kanto Hellquake, a catastrophic disaster that isolates the Kanto region and transforms it into a lawless wasteland. When he is first found, he has no memories of his past, his identity a complete mystery even to himself. It is later revealed that he is the reincarnation of Akira Fudo, the protagonist of Devilman, resurrected in a world that serves as a divine punishment for the demon lord Satan. This connection to a larger cosmic struggle informs his existence, though he rarely speaks of it directly and often seems to act on pure instinct.

Physically, Jack is an imposing and inhuman figure, standing between seven and twelve feet tall with a frame of gorilla-like musculature, wolf-like fangs, and wild, shaggy hair. His eyes are often described as blazing with a primordial, fiery energy. He wears tattered clothing, with a yellow scarf being his only consistent garment. His name is derived from his signature weapon, an oversized jackknife roughly forty centimeters long, which he conceals and brandishes with lethal efficiency. He also possesses the ability to manifest in three distinct forms: his standard gigantic male form, a powerful female form known as Lady Jack who wields a whip or blades, and a muscular adolescent form called Kid Jack. These three forms are later revealed to be fragmented aspects of his true Devilman identity, and merging them restores his original power for the final confrontation.

In terms of personality, Violence Jack is a force of nature rather than a conventional hero. He is ruthless and relentless in combat, rarely speaking and showing no concern for collateral damage or the safety of bystanders. His presence alone can incite violence or cause strange phenomena, such as mysterious earthquakes. However, despite his brutal and often terrifying demeanor, he consistently channels his violence towards aiding the weak. He selectively intervenes on behalf of vulnerable individuals, such as orphans or oppressed communities, but typically only after observing that those victims possess the will to survive and fight for themselves. He expects nothing in return for his help; his assistance is framed as a necessary counterforce against the greater evils that roam the wasteland. His unpredictable nature means that innocent people can be injured or killed as a consequence of his vicious fighting style, placing him firmly in the role of an antihero. He does not seek to make friends or allies, preferring to act as a brutal balancing agent in a world where only strength ensures survival.

His role in the story is as a catalyst for change in the post-apocalyptic hellscape of Kanto. He appears to help the better-willed factions in their battles against tyrannical warlords and violent gangs, fighting enemies such as the samurai warlord known as the Slum King, the biker gang Hell's Wind, and the factions within Evil Town. Rather than leading armies, he often acts as a solitary weapon, destroying the immediate threat and then vanishing without a trace. His intervention is frequently sought out by those he has protected, such as the boy Saburo Tenma or the young leader Takuma Ryu, who are inspired by Jack to become strong commanders in their own right. Similarly, he works alongside individuals driven by revenge, like Jun from Hell's Wind. Despite his solitary nature, his existence is intrinsically tied to his rivals, particularly the Slum King, a tyrant who rules much of Kanto and whose own twisted history contains figures from Jack's past life, such as the limbless, tormented Ryo Asuka and Miki Makimura.

Throughout the narrative of the manga, Jack undergoes a significant development that transcends his initial portrayal as an amnesiac brute. While he appears static and emotionless in many of his encounters, the overarching story reveals that his journey through the wasteland is one of self-discovery. His fragmented memories of his life as Akira Fudo gradually surface, and his three separate forms are shown to be parts of a greater whole. His ultimate purpose is not merely to wander and punish evil, but to gather the strength and will of humanity to fight back against Satan. In the climax of the original manga, he sheds his identity as Violence Jack entirely, merging his three forms to become Devilman once more for a final, decisive battle. This transformation from a mysterious avatar of violence into a complete being fighting for the future of the world represents the core of his character arc.

His notable abilities are rooted in his demonic heritage and superhuman physiology. He possesses immense physical strength, allowing him to perform feats such as hauling anchors with ease, sinking islands, or battling large groups of enemies single-handedly. His speed is sufficient to blitz and catch up with fast-moving motorcycles. He is incredibly durable and possesses an accelerated regenerative healing ability that enables near-instant recovery from severe wounds. In addition to his raw power and his skill with his massive jackknife, Jack is also capable of generating powerful hallucinations to torment or manipulate those he encounters. A golden bird is frequently seen accompanying him, acting as a guide that informs him of where his help is needed. These powers, framed as divine punishment or as the abilities of a reborn Devilman, make him an unstoppable executioner in the lawless world of Kanto.