Kōshū Okumura is a first-year student at Seidou High School who joins the baseball team as a catcher. He originally hails from Kanagawa and previously played for the Daikyou Senior middle school team. His experience with a boisterous coach during that time left him with a strong distaste for loud and overly energetic attitudes. Okumura has a quiet, serious, and analytical personality. He approaches everything with utmost seriousness and initially holds the belief that it is impossible to play baseball with full dedication while also genuinely enjoying the sport. This internal conflict drives his decision to enroll at Seidou, where he hopes to discover whether such a balance is achievable and to test the limits of his own abilities.

Despite his reserved demeanor, Okumura is remarkably outspoken and unafraid to voice his opinions, especially regarding the pitchers on the team. He frequently critiques their performances and does not hesitate to confront even older teammates, such as when he scolds Eijun Sawamura for being too loud in the dining hall. This directness reflects his high standards and his deep commitment to the game. He is a thinking player who relies on a systematic process of elimination and prediction to minimize risk and create favorable situations for his team.

As a rookie catcher, Okumura demonstrates exceptional technical skill. He possesses a refined ability to catch even the most unpredictable pitches, such as Sawamura's innovative "Numbers" repertoire, using soft, full-body movements that help sway the umpire's judgment—a framing technique that gives his pitchers an advantage. He is also a keen strategist who can calmly analyze a pitcher's condition and read a batter's tendencies, allowing him to call games with precision. His high baseball IQ is complemented by his willingness to speak his mind, making him a valuable asset behind the plate.

In terms of key relationships, Okumura initially clashes with Sawamura due to the pitcher's exuberant personality, but he gradually becomes fascinated by Sawamura's potential and develops a complex partnership with him. He becomes Sawamura's primary catcher during practices and many relief appearances, and their collaboration deepens over time. Okumura shares a strained but respectful dynamic with Kazuya Miyuki, the team's star catcher and captain. He views Miyuki as an obstacle he must surpass, and being Miyuki's roommate only intensifies his drive to improve. He is also close to Takuma Seto, a fellow first-year who came from the same middle school, and the two often share scenes together.

Okumura's character undergoes a notable evolution throughout the series. He starts as a detached, skeptical observer who keeps others at arm's length. As he integrates into Seidou's team culture, he learns to appreciate the emotional connection between a pitcher and catcher and begins to understand that serious effort and enjoyment are not mutually exclusive. He grows into a dedicated member of the first-string squad, earning the trust of his teammates and coaches. His sharp tongue remains, but it becomes tempered by a growing sense of belonging.

Okumura's abilities extend beyond catching. He is a competent batter with a sharp, compact swing, and he excels at reading opposing catchers' patterns to find gaps. His reaction time and arm strength are well-regarded, and his overall defensive reliability makes him a strong candidate to eventually take over as Seidou's starting catcher after Miyuki graduates. Through his journey, he represents the next generation of talent striving to uphold the team's legacy while forging his own path.