Isami Nishimura first appears as a high school pitcher displaying a somewhat conceited attitude, frequently overestimating his own worth and baseball abilities. He throws a decent curveball but is not considered a significant threat by the main characters, often being easily forgotten or ignored despite his self-perceived importance. During high school, he fixates solely on Akio Nitta as his rival, viewing this competition as central to his athletic identity.

His personal life includes an unrequited crush on Minami Asakura, who does not return his feelings. Concurrently, he is pursued by Suzuko, a plump girl whom he typically rejects, though he shows occasional concern for her well-being when she faces bullying.

Decades later, depicted in "Mix," Isami has matured into adulthood and serves as coach of his former high school's baseball team. He is married and has a son, Takumi Nishimura, whom he raised to become a baseball prodigy. Isami takes pride in Takumi's physical resemblance and inherited pitching talent, viewing him as an extension of his own legacy. However, he finds himself mildly annoyed by Takumi's boastful personality and persistent pursuit of an uninterested girl—behavior uncomfortably mirroring Isami's own "pathetic" high school experiences.

Despite retaining traces of his earlier pomposity, Isami demonstrates personal growth, displaying a slightly humbler demeanor in his coaching role. His observations of Takumi's struggles evoke self-reflection, particularly regarding his younger self's overconfidence and romantic missteps.