Jack Hanma is the first son of Yuujiro Hanma and older half-brother of Baki Hanma. His mother, Diane Neil, was a secret agent assigned to eliminate Yuujiro during the Vietnam War. After her attempt failed, Yuujiro raped her, resulting in Jack's birth. Unlike Baki's mother, Diane survived and raised Jack. Driven by a desire to surpass Yuujiro and avenge his mother, Jack engaged in extreme overtraining from childhood. This regimen deteriorated his muscles, leaving him frail and incapable of gaining strength through conventional means despite years of effort. A turning point came when Jack met Dr. John, a scientist inspired by Yuujiro. Dr. John provided experimental steroids called X-4, promising enhanced physical capabilities. Despite warnings of potential fatality, Jack endured excruciating pain and survived due to his Hanma lineage. The steroids granted him immense strength and muscle mass but caused severe mental instability, addiction, and a reduced lifespan. This transformation enabled him to kill a polar bear with his bare hands, emulating Yuujiro's feat with greater brutality. Jack debuted in the Maximum Tournament Saga under the alias "Jack Hammer" to conceal his identity. Before his first match, he encountered Kozue Matsumoto, Baki's girlfriend, and flirted with her, provoking Baki's anger. In the tournament, Jack dominated opponents: he swiftly defeated Brazilian grappler Sergio Silva in Block C, overpowered Kengo Misaki in the second round, and advanced past Alexander Gallen in the quarter-finals. His semi-final bout against Gouki Shibukawa showcased tactical ingenuity; after being thrown multiple times by Shibukawa's Aiki techniques, Jack bit Shibukawa's arm, exploiting a weakness in the defensive style to secure victory. This win led to the finals against Baki. Jack initially overwhelmed his half-brother but ultimately lost after Baki activated his Demon Back. The defeat prompted a shift in Jack's demeanor, making him more humble and amicable toward Baki. Physically, Jack is exceptionally tall and muscular. His initial height was 193 cm (6'4"), increasing to 213 cm (7'0") post-Maximum Tournament due to limb-lengthening surgery performed by Kureha Shinogi. After a later fight with Pickle, he underwent further surgery, reaching 243 cm (8'0"). His teeth were shattered by Izou Motobe and replaced with titanium implants. This led him to develop "Goudou," a martial art centered on biting for its practicality and accessibility. Jack's fighting approach combines brute force with tactical intelligence, utilizing pressure-point strikes and mimicking Aiki principles. He refuses to kill downed opponents, even when provoked. Jack's personality is defined by relentless determination and obsession. His early weakness forged a resolve to become stronger at any cost, fueling his steroid use and surgical modifications. Post-transformation, he exhibited animalistic aggression, often biting opponents and fighting without restraint. His fixation on defeating Yuujiro sometimes led to overconfidence. After losing to Baki, he displayed greater emotional control, engaging in calm activities like feeding koi fish. He maintains complex relationships: his rivalry with Baki includes mutual respect and shared goals, while his interactions with Kozue suggest a blend of genuine interest and provocation. Jack endures severe injuries without quitting, continuing to fight after having half his face bitten off and waking from a coma. Beyond the Maximum Tournament, Jack's physical and tactical evolution continued. His encounters with Pickle highlighted his persistence; though outmatched, he attacked Pickle's ears in a near-fatal maneuver, causing the caveman to perceive him as a "wasp"—the only creature Pickle feared. This fight reinforced Jack's reputation as a figure who could unsettle even primordial beings. His refusal to yield, despite the toll on his body and lifespan, remains central.