Shintaro Nishihiro is a first-year student at Nishiura High School and a member of its baseball team. Unlike most teammates, he only began playing baseball upon entering high school, having participated in track and field during middle school. His interest sparked in his third year of middle school after reading a baseball manga, leading him to join despite initial reluctance about starting late. Academically gifted, Nishihiro consistently achieves high test scores without apparent studying, ranking among the smartest first-year students. Physically, he has short-medium black hair with greenish tones and green eyes, standing 1.70 meters tall and weighing 60 kilograms.

On the baseball team, Nishihiro primarily serves as a reserve outfielder and bench player. Due to the team's limited roster, he frequently acts as the third-base coach during offensive innings. His lack of prior experience confines his on-field appearances mostly to left field, where he demonstrates noticeable but gradual improvement through intensive training. Despite his reserve status, he remains motivated and committed to supporting the team from the bench, understanding his role contributes to the team dynamic. His classmate and close friend Kazutoshi Oki joined the baseball club specifically to avoid being alone, indicating Nishihiro's influence. He maintains a close relationship with his younger sister, often seen playing with her at home. His surname Nishihiro combines "west" (西) and "wide, broad" (広), while his given name Shintaro incorporates "dragon" (辰), "thick, big" (太), and "son" (郎).