Hiromitsu Ochiai serves as the assistant coach for the Seidou High School baseball team, brought in by the school's administration with the expectation that he will eventually succeed the current head coach, Kataoka Tesshin. He brings with him a history of success, having previously coached a school that regularly appeared at the national tournament, Koshien.

Ochiai is characterized by a coaching philosophy that is results-driven, pragmatic, and intensely focused on efficiency. He believes that high school baseball is ultimately about skills and results, and he sees little value in spending time on players he deems to lack the potential to become elite. This leads him to propose radical ideas, such as drastically reducing the team's roster to only its most talented members. He often comes across as arrogant, smug, and calculating, with a tendency to appear suddenly and approach players individually when the head coach is not present to share his perspective. Despite this often cold exterior, he is noted to be shy around strangers and has a young daughter, suggesting a personal life away from the diamond.

Ochiai's primary motivation revolves around building a winning team through the cultivation of exceptional individual talent. From his arrival, his strategy is centered on making pitcher Furuya Satoru the team's undisputed ace and the best pitcher in Japan, believing that the team's success is intrinsically tied to Furuya's development. He is willing to sacrifice team victories in the short term for Furuya to gain invaluable game experience, a philosophy that puts him in direct opposition to Kataoka’s more holistic and team-oriented approach. His long game is to see his vision for the team realized, and he is often depicted as impatiently waiting for the time when he will take full control.

In the story, Ochiai's role is multifaceted. He acts as a foil to Coach Kataoka, creating a fascinating and tense dynamic within the Seidou clubhouse. While Kataoka focuses on mental conditioning and nurturing the potential of all players, Ochiai is a technical specialist who excels at improving players' skills. He is responsible for pushing along the arcs of several characters, notably helping Sawamura Eijun develop breaking balls like the change-up after initially dismissing him as unusable due to the yips. He also works with second-string players, offering them technical advice to improve. However, his methods are not always benevolent; some of his early actions, such as trying to convert a struggling Sawamura into a sidearm pitcher for batting practice, are seen as manipulative and potentially destructive.

His key relationships define much of his role in the series. With Furuya Satoru, Ochiai is a dedicated mentor who sees the pitcher's raw talent as the key to greatness, constantly advocating for his increased usage and focused development. His relationship with Sawamura Eijun is more complicated, beginning with open skepticism and a willingness to use him as a tool for the team's benefit, but evolving as he contributes to Sawamura's technical growth and admits he underestimated his potential. The most significant relationship is with Head Coach Kataoka Tesshin. While they clash ideologically, with Ochiai viewing the team as a vehicle for its star player and Kataoka viewing it as a collective unit, there is a grudging respect between them. Ochiai is curious about Kataoka's methods, and despite their disagreements, Kataoka does value Ochiai's technical opinions as a coach.

Ochiai undergoes a subtle but notable development throughout the series. He begins as a character who seems primarily villainous, designed to make Kataoka look better by comparison. However, he proves to be a brilliant instructor, and his impact on the team is largely positive. He starts to learn the value of Kataoka's motivational tactics, observing how the head coach inspires the players and even envying the loyalty and trust he commands from the team. While his core philosophy remains results-oriented, his experiences at Seidou force him to appreciate that personal growth and team growth are intertwined, and he moves from being a pure plot device to a more complex figure who contributes significantly to the team's strength.

Ochiai's notable abilities lie not on the field, but in his profound technical knowledge of baseball. He has an encyclopedic understanding of the game and a sharp eye for player mechanics and potential. He can identify the most suitable pitches for a pitcher's unique style, as seen when he immediately recognized the changeup as the ideal breaking ball for Sawamura. His strength is in providing simple, direct advice that leads to rapid and measurable improvement in a player's skills, cementing his role as a valuable, if controversial, asset to the Seidou baseball team.