Hōsaku Samon, the eldest of six children, was born into a poor farming family in Kumamoto Prefecture. His father died from overwork when Samon was in fourth grade, followed shortly by his mother succumbing to exhaustion, leaving the children orphaned. Distant relatives took them in but subjected them to harsh labor in the fields. This difficult upbringing inadvertently forged Samon's physical strength. His relentless studying by moonlight, due to restricted access to electric lights, severely damaged his eyesight, resulting in extreme nearsightedness and his trademark glasses.

Samon's introduction to baseball came when a stray home run ball flew toward him outside a stadium. Instinctively using a bat, he hit the ball back to its origin, impressing witnesses Hyūma Hoshi and Chūta Ban. This event ignited his interest in the sport. He enrolled in Kumamoto Nōrin High School with a tuition exemption, promising his siblings he would handle their hardships. As team captain and cleanup batter, he led the school to the Kōshien Tournament for the first time in three years. In the semifinals against Hyūma Hoshi's team, Samon's powerful hit shattered his bat upon contact; the broken piece injured Hyūma's thumb, contributing to Seiun High's subsequent loss in the finals.

Despite Kumamoto Nōrin's loss, Samon's performance attracted professional scouts. Though the Yomiuri Giants, managed by his prefectural compatriot Tetsuharu Kawakami, pursued him, Samon chose the Taiyō Whales instead. His decision mirrored rival Mitsuru Hanagata's earlier rejection of the Giants, as both sought to compete directly against Hyūma. Upon turning professional, Samon relocated his siblings from their relatives' home to an apartment near the Tama River, reuniting the family. His primary motivation became defeating Hyūma, driving him to meticulously study Hyūma's pitching patterns and weaknesses. This analytical approach defined his career; he compiled detailed scouting reports known as "Samon's memo," cataloging opposing pitchers' habits and specialties across the league.

Samon's personal life intersected with his rivalry when he encountered Kyōko, leader of the delinquent "Tornado Group," during frustration after Hanagata hit a home run off Hyūma's signature pitch. Wrongfully accused of molestation by the group, he was rescued by Hyūma. Samon developed feelings for Kyōko, who initially showed interest in Hyūma. Later, as Hyūma faced career-ending injuries from his forbidden pitches, he deliberately alienated Kyōko to preserve Samon's chance with her. In a final letter before retirement, Hyūma urged Samon to confess to Kyōko and revealed the truth behind his behavior. Kyōko accepted Samon's proposal, and they married in 1971. The wedding was attended by close friends and family, including rivals Hanagata and Ban, but no representatives from Samon's professional team.

After Hyūma's temporary disappearance from professional baseball, Samon remained active with the Taiyō Whales. When Hyūma returned as a substitute hitter, Ban requested Samon's scouting notes to aid his transition. Samon initially refused but relented after deducing Hyūma's involvement, providing the memo which included data on pitchers like Senichi Hoshino's knuckleball and Yoshiro Sotokoba's shoot. Samon later resumed his analytical focus when Hyūma returned as a right-handed pitcher, reestablishing their competitive dynamic.