Sōichirō Izumi is the master of the fictional Takemiya martial arts school, a unique style blending Karate, Judo, Aikido, Greco-Roman Wrestling, and Jujutsu. The school emphasizes generational knowledge transfer, with masters designating official heirs. Its signature technique, the Tiger King, involves intricate leg placement around an opponent's neck and chin to set up an armlock, adapted by each practitioner.

Izumi mentored Juzo Fujimaki, forging his formidable skills through rigorous training at his home. Fujimaki developed a close relationship with Izumi's daughter, Saeko. A pivotal incident occurred when Fujimaki discovered an intruder assaulting Saeko in Izumi's absence. Enraged, he killed the assailant using Takemiya techniques, witnessed by Saeko. Misinterpreting her traumatized reaction as horror towards his violence, Fujimaki fled, becoming a fugitive. This severed their master-student bond and left Saeko's assault unaddressed.

Years later, Izumi accepted Bunshichi Tanba as a student. Tanba, a notorious dojo-crusher seeking to prove himself after a defeat, had previously fought Izumi in a challenging match where Izumi demonstrated significant resilience against Tanba's strikes despite his advanced age. Izumi also trained Tsutomu Himekawa, a protégé from the Hokushin Karate school.

After returning from an underground tournament, Fujimaki sought to rejoin Izumi's tutelage and vie for the position of Takemiya successor against Tanba and Himekawa. His motivations included reclaiming his place and reconnecting with Saeko, whose unresolved feelings lingered. Izumi's role evolved to oversee this contentious dynamic among his three top students, balancing mentorship with the school's legacy.

Izumi exudes stoicism and dedication to martial arts philosophy. His physical portrayal emphasizes maturity and muscularity, reflecting his discipline.