Eijirō Sakaki previously coached the Seidou High School baseball team, guiding them to multiple National tournament appearances. His coaching leveraged deep baseball knowledge, especially in strategic calls like hit-and-runs and bunts, with players acknowledging his keen sense for timing these plays perfectly.

After Seidou, Sakaki advanced to coach at Hosei University before returning to high school baseball as the head coach of Yura Technical High School in West Tokyo. This role placed him in direct competition with his former protégé.

Sakaki shares a significant mentor history with Seidou's current coach, Kataoka Tesshin. Sakaki personally recruited Kataoka during high school when Kataoka, nicknamed "Demon Child" for his volatile temper, lacked discipline and clashed with seniors. Sakaki saw past this to Kataoka's fierce competitive spirit and resilience, ultimately making him the team's ace. This trust proved pivotal as Kataoka matured into a trusted leader who steered Seidou to the Summer Nationals finals.

During a visit to observe an intra-squad game at Seidou, Sakaki approached Kataoka. He formally announced his return to high school coaching at Yura Technical and directly challenged his former student, declaring, "I came to declare war on you, Tesshin." Emphasizing his limited time before retirement, Sakaki stated, "I don't have much time left [till retirement] to work as a coach, so... Don't run away, Tesshin," underscoring his intent to compete against Kataoka.