Ren Mihashi endured intense bullying as Mihoshi Middle School's starting pitcher. Teammates resented his position, suspecting favoritism due to his grandfather owning the school, and believed reserve pitcher Shūgo Kanō was more talented. Despite relentless harassment—including teammates deliberately playing poorly and his catcher refusing to give pitch signs—Mihashi stubbornly remained on the mound for every game. This period amplified his existing nervousness and shattered his self-confidence, convincing him he caused all losses. He transferred to Nishiura High School intending to quit baseball, though he retained a deep love for pitching.

At Nishiura, coach Maria Momoe recruited him after learning of his pitching background, as the team lacked a dedicated pitcher. Mihashi became the ace despite initial reluctance and intense anxiety. His technical pitching skills were immediately notable, featuring exceptional control that divided the strike zone into nine sections—a precision exceeding the professional standard of four. His repertoire included four breaking balls, notably an unconventional fastball confusing batters with unusual movement and a dropping slider. While his pitch speed started slow, training under Momoe gradually increased his velocity. He later learned proper fastball technique with backspin and began pitching from the windup position.

Mihashi's personality is marked by extreme anxiety, pronounced lack of self-confidence, and a tendency to stutter or struggle with communication, especially under stress, causing early friction with his primary catcher, Takaya Abe. Despite this, he exhibits intense stubbornness and dedication to pitching, refusing to leave games even when exhausted. He initially resisted having a reserve pitcher, insisting on his role as the sole pitcher. Off the field, he is described as gullible, enjoys eating, and holds a baseball even while sleeping. Academically, his grades are poor, requiring significant effort to enter Nishiura. He demonstrates surprising confidence and skill in cooking. He also harbors a fear of dogs and dislikes the nickname "Renren" given by his cousin, Ruri Mihashi, with whom he lived during middle school.

His relationships form a core aspect of his development. His partnership with catcher Takaya Abe began strained due to Abe's short temper and Mihashi's fear of displeasing him. Mihashi deeply respected Abe, crediting him entirely for pitching success and relying on his game-calling. Abe valued Mihashi's work ethic and control. Their dynamic evolved into greater mutual understanding, with Abe promising to stay healthy to catch all Mihashi's pitches. Mihashi shares a close bond with Yūichirō Tajima, who intuitively understands him with minimal words and acts protectively. Mihashi maintained childhood friendships with Yoshirō Hamada, who gifted him his first glove, and Kanō, whom he now calls "Shuu-chan." Their rivalry transformed into mutual respect, with Kanō acknowledging Mihashi's superior pitching; both now aim for Koshien as renewed rivals. His relationship with cousin Ruri also improved, evidenced by inviting her to watch his high school games—contrasting his middle school reluctance.

Across official media, Mihashi undergoes considerable development. While fundamental anxiety persists, his pitching confidence grows significantly through Nishiura teammates' support and game successes. He becomes more willing to express thoughts to Abe and the team. His pitching skills advance under Momoe's coaching, gaining speed and mastering new pitches. His emotional resilience strengthens; he confronts his past during a practice game against Mihoshi, accepting former teammates' apologies but choosing to stay with Nishiura. The manga depicts him actively embracing his rivalry with Kanō after Kanō messages him following a loss, signaling a healthier competitive spirit. Mihashi's journey centers on overcoming trauma, gradually trusting others, and finding belonging through baseball.