Tsutomu Ishii anchors the Mizuho High School basketball team as their power forward. Standing 190 cm tall, he dominates the paint as one of the team's tallest players, leveraging his significant height and top-tier jumping ability to control plays under the basket. His basketball journey began at Takakura Junior High, forging a close, enduring friendship with teammate Kenji Dobashi that continues at Mizuho. Early in high school, Ishii's highly volatile temperament defined him; his extremely short-tempered and hot-blooded nature often triggered self-destructive behaviors during games, leading to frustration-induced fouls. This made him a liability, notably during his sophomore year at the Kanto Rookie Tournament.

Ishii undergoes substantial growth through rigorous training and experience, significantly improving his skills and mental resilience. By his senior year, particularly in the Inter-High tournament, he demonstrates maturity by encouraging nervous underclassmen and maintaining composure in critical moments. This development stabilizes his performance, allowing him to play full-time through the tournament's third round without being benched.

On the court, Ishii excels in rebounding, blocking, and dunking, maximizing his height and athleticism. While lacking a consistent mid-range shot, he maintains a relatively high free-throw percentage. His exceptional stamina and endurance rival top teammates, coupled with iron-like durability. Off the court, Ishii favors casual attire, frequently wearing Converse shoes. His interests include listening to bands like Unicorn, The Blue Hearts, Lindberg, and some Western music. His favorite food is yakiniku; he favors sports as a subject and dislikes English. Ishii admires NBA player Karl Malone and prefers mature, cheerful personalities in romantic relationships.

His relationship with Dobashi, rooted in shared history and mutual respect, remains central to his character. Their dynamic strengthens Mizuho's cohesion during high-pressure games.