Though Yamaguchi played basketball for three years, he decided to do something different in high school. Ishigaki Koutarou scouted Yamaguchi for the cycling club back when Yamaguchi was a first-year trying to decide on a club. He was initially not very confident about cycling, but Ishigaki convinced him to join in by pushing the idea of it being a "team sport."

41st Summer Inter High

Day 1

Day 2

Yamaguchi is one of the sprinters left behind when Midousuji Akira acts "phase 13" of his plan on the second day of the Inter High. He retires from the race along with Ihara Tomoya.

42nd Summer Inter High

Yamaguchi agonizes over the responsibilities placed on him by both Ishigaki and Midousuji. Ishigaki asked Yamaguchi to act as Midousuji's "conscience," while Midousuji ordered him to be part of his plans. Yamaguchi feels as if he cannot live up to the expectations placed on him because he's not that kind of person. When Midousuji tells him to go after Onoda Sakamichi, he is surprised by Onoda's strange aura. Yamaguchi calls out for Onoda to wait and Onoda actually does slow down to listen. He tells Onoda that refusing Midousuji's offer to work together was good judgement, as Midousuji actually just wants to defeat Sohoku. Yamaguchi also tells him to go on ahead, and Onoda excitedly thanks him before accelerating.

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