Ittetsu Hoshi was a former third baseman for the Yomiuri Giants whose career ended prematurely due to a shoulder injury sustained in World War II. After returning from combat, he forged the "Devil's Fastball" (魔速球, Ma sokkyū) to counter his diminished pitching ability, but abandoned professional baseball when critics condemned its unsportsmanlike nature. Following his wife Harue's death, he redirected his own unrealized ambitions into rigorously training his son, Hyūma Hoshi. His extreme methods included weighted casts to stunt Hyūma's growth and hitting flaming baseballs at him. This harsh regimen initially fostered resentment in Hyūma toward baseball, though Ittetsu framed it as essential for achieving greatness.

After Hyūma joined the Giants, Ittetsu accepted a coaching position with their rivals, the Chunichi Dragons, deliberately choosing jersey number 84 to complement Hyūma's number 16, symbolizing a pursuit of "perfection" through their conflict. In this role, he trained foreign player Ozuma and later orchestrated a trade to acquire Hyūma’s best friend, Ban Chūta. Ittetsu intended to mold Ban into Hyūma’s rival, subjecting him to similarly brutal training regimens and viewing their confrontation as a necessary battle for Hyūma’s development.

Within the *Shin Kyojin no Hoshi* sequel series, Ittetsu continued his indirect engagement with Hyūma. He once disrupted a grassroots baseball game by exposing a disguised Hyūma, acting as a substitute hitter, and shattered his sunglasses with a thrown pebble. This act forced a confrontation, yet their relationship retained its lifelong dynamic—combative yet rooted in a mutual, albeit strained, dedication. Ittetsu's philosophy emphasized relentless struggle, invoking metaphors like the phoenix rising from ashes to embody perseverance. He consistently prioritized baseball excellence over personal bonds, declaring sentimentality had no place in competition.