|Height||184 cm (6'0½")|
|Weight||70 kg (154 lb.)|
|Hair Color||Dark Blue|
|Manga Debut||051. "Super Problem Child"|
|Anime Debut||021. "Enormous Trouble Makers! Hanamichi vs Miyagi"|
|Japanese Voice||Ryōtarō Okiayu|
|English Voice||Vincent Guiterrez (GMA Dub)|
|[v · t · e]|
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.
Regretful for quitting basketball for 2 years, he has returned with a renewed determination of conquering the nation he once dreamed of. While being competitive against Akagi during their 1st year, he has learned to accept the latter as the suitable captain for the team. He is also not afraid to stand up for a friend, just like what he did for Tetsuo; he insisted Ryu and his gang to beat him up instead of the latter.
While his lack of stamina is his main weakness, his great determination and trust for his team is what keeps him going, shown when he is totally worn out against Sannoh.
He went to Takeishi Junior High, where he was the MVP in his final year. It was at the final match where he met Coach Mitsuyoshi Anzai, who encouraged him not to give up hope until the last second, and enabled him to win the game. As a result, he went to Shohoku High School, having turned down Coach Taoka Moichi (who was also present at that game) when he offered him a place at the more prestigious Ryonan High.
He injured his knee during practice in his freshman year, and exacerbated his injury by returning to play before he had been healed. As a result he was not able to compete in the district tournament, and was disheartened into quitting basketball and becoming the leader of a gang.
He retained a great resentment toward the basketball club for a long time, and he and his gang picked on Ryota Miyagi during the latter's 1st year. A fight ensued, in which Miyagi beat Mitsui senseless, before taking a beating himself, knocking out a few of Mitsui's frontal teeth.
Both of them were in the hospital until after the practice game against Ryonan in Mitsui's third year. At this point, Mitsui attempted to ruin the basketball team by forcing them to fight against his gang and trying to get them banned from the regional tournament for fighting. In the end, Sakuragi's own gang - Yohei, Takamiya, Ohkusu and Noma - along with Norio Hotta and Mitsui's own friends 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 - a promise that he upheld even when he found himself cornered by some members of his old gang outside Shohoku.
Mitsui is best known on the team as a 3-point specialist, though his skills extend to other areas. During a practice match, he managed to shut down Sakuragi, playing Center. His defensive ability completely shut down Fukuda's scoring abilities. His 3-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 Nationals 2nd Round. He can also create a shot for himself, whenever facing his defender.
His major weakness is his lack of stamina due to his long break from playing basketball thus, in the later halves of games, he has not always been reliable as he was before. Like he was completely worn out in the game against Shoyo and Kogure has to sub in for him; Collapsed on court during the game against Ryonan; Totally worn out against Sannoh and can barely move. He has also lost his consistency in shooting 3-pointers and even misses despite being open.
The two players had history with one another before Mitsui stopped playing basketball. He once competed vs Akagi in order to determine who would be captain.
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 2nd half. He managed to land 20 points during the game and cover the 12 point lead of Shoyo before tiring out to continue.
- In the original English Dub of the anime, Sakuragi calls him "Missy" instead of "Mitchi".
- He is considered as one of the problem players because of the fight history. The others are Miyagi, Rukawa and Sakuragi.
- He is likely based on former NBA player, Craig Hodges. They both wore the no. 14 jersey and were three-point shooters.