Shigeyoshi Kumada serves as the go-kart circuit manager and becomes a central figure in Capeta's racing journey. He acts as an unofficial team manager and a dedicated mentor to the young protagonist, gradually filling a fatherly role while providing guidance both on and off the track. A highly skilled mechanic and engineer, Kumada is directly responsible for building, maintaining, and fine-tuning the cars Capeta drives. His approach is meticulous and detail-oriented, insisting on high standards and pushing Capeta to reach his full potential, often through demanding training and exacting technical feedback.

Beneath his strict and sometimes stern exterior, Kumada carries a deep, protective affection for Capeta and invests himself fully in the boy's success. His own past is touched by tragedy and personal regret. Hints of a troubling earlier incident and the guilt he still carries fuel a quiet determination to help Capeta avoid similar pitfalls, making his mentorship as much about redemption as it is about racing ambition. Despite the weight of that past, he remains steady, reliable, and entirely committed to Capeta's dream of reaching professional levels.

Kumada's relationship with Capeta defines his role in the story. He becomes something of a surrogate parent, balancing tough love with unwavering support. He rarely seeks the spotlight or personal recognition, preferring to work behind the scenes and let results on the circuit speak for themselves. Through his mechanical expertise, methodical problem-solving, and emotional resilience, he proves to be an indispensable foundation for Capeta's growth, adapting his guidance as the challenges of motorsport become ever more demanding.