Jack Hanma, son of Yuujirou Hanma and Canadian UN operative Diane Neil, was conceived when Yuujirou raped Diane during the Vietnam War. Learning of this trauma in childhood, Jack dedicated his life to surpassing his father in strength to avenge his mother. This obsession fueled extreme overtraining from a young age, pushing his body to physical collapse – exemplified by training until sweat-covered on a church floor at age ten under the supervision of his paternal figure, Father Samuel. His relentless regimen caused severe muscle deterioration, reducing him to a skeletal state. To counteract this, he accepted experimental steroids (X-4) from Yuujirou-inspired scientist Dr. John. These drugs granted immense power but induced mental instability, physical addiction, and a drastically reduced lifespan. Despite warnings, Jack demanded higher doses, enabling brutal feats like bare-handed polar bear killing. Physically, Jack underwent multiple surgical enhancements. Initially 193 cm (6'4") and 116 kg (255 lbs), he received limb-lengthening surgeries from Dr. Kureha Shinogi after the Maximum Tournament, reaching 213 cm (7'0") and later 243 cm (8'0") at 211 kg (465 lbs). He possesses blonde hair inherited from his mother, blue eyes, and extensive training scars. After losing all his teeth fighting Izou Motobe, he replaced them with titanium dentures. In the Maximum Tournament, he fought as "Jack Hammer" to conceal his identity. His matches included swiftly defeating Sergio Silva via uppercut and advancing using his signature biting technique. He confronted half-brother Baki Hanma in the finals, fighting through severe bodily stress from steroid abuse. After losing, he acknowledged Baki's victory respectfully. He later faced Yuujirou but was easily defeated. Jack's personality evolved from single-minded vengeance to measured determination. Initially displaying animalistic brutality that startled opponents like Pickle (who viewed him as a persistent "wasp"), he occasionally showed hidden warmth, such as feeding koi fish with Tokugawa or subtly caring for Baki. Losses to Baki, Pickle, and Motobe fostered a calmer, more strategic approach. The Motobe defeat, which shattered his teeth, prompted him to formalize his biting focus into the martial art "Goudou." Post-tournament, Jack appeared in conflicts involving death row convicts, Pickle, and Musashi Miyamoto. His ongoing quest to defeat Yuujirou remained central, driving his physical transformations and refinement of Goudou.