Jack Hanma is the firstborn son of Yujiro Hanma, making him the older half-brother of the series' protagonist, Baki Hanma. His birth was the result of a violent encounter between Yujiro and a Canadian mercenary and spy named Diane Neil. After being discovered as an agent, Diane was assaulted by Yujiro and subsequently imprisoned, where she gave birth to Jack. This tragic origin forged in him a single, all-consuming motivation: to grow strong enough to defeat his father, Yujiro, and avenge his mother’s suffering.

Jack's personality is defined by an almost inhuman level of focus and sacrifice. He is not driven by a desire for fame or the simple joy of fighting, but by a cold, obsessive need for power. His most defining characteristic is his complete willingness to abandon his own well-being, future, and even his lifespan for the sake of becoming stronger. This philosophy is best captured in his own words: he has no intention of living a long life and is prepared to exchange death for a strong body. Despite this brutal dedication, Jack is not portrayed as a mindless brute. He possesses a quiet, reserved demeanor outside of battle and has shown moments of genuine respect and even brotherly affection, most notably towards Baki after their fierce battle in the underground arena.

In the story, Jack first emerges as a formidable and mysterious participant in the underground Tokyo Dome fighting tournament. His aggressive and savage fighting style, which often involves biting his opponents, quickly marks him as a major threat. His role is initially that of a final obstacle for Baki, a rival who shares his blood and a similar goal. As the series progresses, Jack remains a persistent force, constantly seeking out the world's strongest fighters to prove himself. His key relationships are fraught with intensity. His relationship with his father Yujiro is one of bitter resentment and a desperate desire for acknowledgment, though Yujiro largely views him as a failure compared to Baki. His dynamic with his brother Baki is complex, evolving from pure rivalry into a relationship of mutual respect, even as Jack continues to walk a darker, more self-destructive path to power.

Jack's personal development is a chronicle of extreme, physical metamorphosis. He was not born with the natural, godlike talent of his father or brother. Instead, he has forcibly reshaped himself through science and suffering. To overcome his limits, he became a test subject for a scientist named Dr. John, ingesting massive quantities of experimental steroids that ravage his body but grant him tremendous strength. After his initial defeat by Baki, he underwent excruciating bone-lengthening surgery to increase his height and reach, becoming a towering giant. His journey is one of constant, painful augmentation, culminating in the replacement of his teeth with titanium alloy implants to perfect his unique combat style.

Jack's notable abilities center on his immense physical power and his signature technique: the bite. He has developed an entire martial arts system around biting, known as "Goudou" (The Biting Way), which allows him to target and sever an opponent's vital arteries and muscles with his powerful jaws. While his strength, durability, and sheer size are world-class, allowing him to trade blows with giants like the primitive man Pickle, he lacks the supernatural "Demon Back" that signifies a true prodigy of the Hanma bloodline. To compensate, he relies on a terrifying combination of raw power, relentless tenacity, and a high-level tactical understanding of anatomy and martial arts, having once even copied the principle of Aiki mid-fight. He stands as a testament to what can be achieved through sheer, unyielding will, even at the cost of one's own humanity.