Jack Hanma is the first son of Yuujiro Hanma, born after Yuujiro raped Canadian UN operative Diane Neil during the Vietnam War. Unlike Baki's mother, Diane survived and raised Jack. Learning his origins, Jack developed an obsessive desire to surpass his father through combat. As a child, he engaged in extreme overtraining that damaged his body, leaving him frail despite relentless effort. This included punching sandbags for over 12 hours without rest, causing muscular deterioration. A turning point came when scientist Dr. John, inspired by Yuujiro, provided experimental steroids called X-4. These granted immense strength but caused mental instability, bodily degeneration, and a reduced lifespan. Jack became addicted, ignoring fatal warnings, and ultimately drove Dr. John to suicide. The physical transformation enabled feats like killing a polar bear barehanded, albeit brutally. Jack's appearance drastically changed due to surgical interventions. Initially 193 cm (6'4") and 116 kg (255 lbs), he underwent limb-lengthening surgeries after the Maximum Tournament, reaching 213 cm (7'0") and 160 kg (352 lbs). Post-fight with Pickle, further surgery increased his height to 243 cm (8'0") and weight to 211 kg (465 lbs). He possesses blonde hair, blue eyes, and extensive scarring from training. After losing all his teeth to Izou Motobe, he received titanium dentures. His physique resembles his grandfather Yuuichirou Hanma, with facial features akin to Yuujiro. In combat, Jack combines brute force with tactical ingenuity. His signature style, Goudou, centers on biting—a technique developed after realizing pure strength was insufficient. This leverages jaws capable of crushing bone, inspired by the idea that even the weak could defend themselves with bites. Techniques include targeting vital throat points and striking ears to pierce the brain. He adapts mid-fight, like countering Shibukawa's Aiki after repeated throws. Key battles involve defeating convicts like Sikorsky, enduring Mohammad Alai Jr.'s assaults while severing his tongue, and fighting Pickle until the caveman retreated in fear. Despite aggression, Jack refuses to kill downed opponents. Jack's personality is defined by relentless determination and sacrificing longevity for power. Early steroid use exacerbated instability, fueling single-minded revenge. He views Baki as a rival but harbors underlying familial concern, evident when flirting with Baki's girlfriend Kozue to provoke him. After losing to Baki and Motobe, Jack matured, adopting a calmer demeanor and refined manners, like feeding koi fish with Tokugawa. His intelligence, likely inherited from the Hanma lineage, surfaces in strategic fighting innovations. Initially dismissive of martial arts philosophy, his creation of Goudou reflects a shift toward structured combat discipline. Jack's development shows progression from self-destructive obsession to measured resilience. He trains beyond physiological limits, embracing multiple life-threatening surgeries and steroid regimens. Post-defeats, he channels ferocity into refining Goudou, symbolizing a balance between primal aggression and disciplined artistry. His rivalry with Baki evolves into mutual respect, though his primary drive remains surpassing Yuujiro, regardless of the cost to his body or lifespan.