Before joining Shohoku, Mitsui Hisashi was an MVP for Takeishi Junior High. Despite having opportunities to join more prestigious high schools like Ryonan, he choose to go to Shohoku because of Coach Anzai; Anzai had given him the courage to win in the Junior High tournament by encouraging him during the last seconds of the match. However, an injury to his left knee in a training session interrupted his basketball career. An imprudent early return to the basketball court reinjured the knee more severely, leading to a basketball hiatus of over a year.
Although he eventually healed, he was embittered by his exclusion from the court, and Mitsui quit basketball and became the leader of a gang. His gang got in a fight with Miyagi, and during the fight, Miyagi beat Mitsui senseless before taking a beating himself.
Afterwards, Mitsui attempted to ruin the basketball team by instigating them into fighting with his gang, as the team would have been banned from competition for fighting. In the end, Sakuragi’s own gang—Yohei, Takamiya, Ookusu and Noma—voluntarily took the blame for the fight, and Mitsui was filled with remorse when he saw Anzai again. He rejoined the team and vowed never to fight again.
Mitsui is best known on the team as a three-point specialist, though his skills extend to other areas; during a practice match, he once managed to shut down Sakuragi, playing center. His defensive ability completely shut down Fukuda’s scoring abilities. His three-pointers helped Shohoku catch up when they were 12 points down against Shoyo, and again when the team was 20 points down against Sannoh in the National Championships. His greatest weakness, though, is his lack of stamina due to his long break from playing basketball. Thus, in the latter halves of games, he has not always been as reliable—he missed a potential game-winning shot against Kainan and collapsed on-court during the game against Ryonan.