He was an MVP in his final year at Takeishi Junior High.Despite having the opportunity to join more prestigious high schools like Ryonan, he choose to go to Shohoku because of Coach Mitsuyoshi 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 in his first year at Shohoku interrupted his basketball career. An imprudent early return to the court reinjured the knee more severely, leading to a basketball hiatus of over a year. Although he eventually healed, Mitsui was embittered by his exclusion from the court and quit basketball to become the leader of a gang. His gang picked on Ryota Miyagi during his first year, and during a fight that ensued Miyagi beat Mitsui senseless before taking a beating himself, knocking out a few of Mitsui's frontal teeth and leaving the both of them hospitalized until after the practice match against Ryonan.
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, Ohkusu 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 was completely worn out in the game against Shoyo, missed a potential game-winning shot against Kainan, and collapsed on-court during the game against Ryonan.
Hasegawa lost against Mitsui during Middle School and subsequently lost his confidence. But the time he saw him being a delinquent, he is in shock after seeing Mitsui's drastically changed behavior. Seeing each other again in the match against Shohoku and Shoyo three years later, he marks him with a box-and-one strategy. Mitsui overhears Hasegawa tell a Shoyo player that he won't allow him more than five points. This reinvigorated Mitsui's MVP talents and he consecutively shot several three points on the second half. He managed to land 20 points during the game and cover the twelve point lead of Shoyo before he was too tired to continue to play.