First Manga Appearance: Chapter 06
Name Meaning: Tsukai=errand; messenger, Tsuyoshi=powerful; strong
Gangs: The Four Emperors of Enoshima P.S. 4; Oni-baku Gumi
Homeroom Class: 1-A; 2-A
Current School: Tsujido High School
Previous Schools: Enoshima Junior High School
Notable Features: Conveyed as very righteous
Attended Enoshima Junior High & was known as one of the 4 Emperors of Enoshima P.S.4, denoting that he was one of the 4 strongest in the school. He is a strong fighter & is not afraid to go up against opponents that are stronger than him. His senpai at Enoshima Junior High was Kunito Nakajo, allowing him to gain in depth inside knowledge of various gang-related information, particularly information about Midnight Angel.
He currently attends Tsujido High & quickly befriends the Oni-baku upon their arrival & aids them in their quest for girls with Makoto, including hooking them up with Keiko & Mayumi, informing them about the Culture Festival at the Ferris School for Girls & the annual Valentine's Day parade. He also is willing to lend his friends a hand, such as offering to help Eikichi take revenge on Machida.
When the Kamakura Mad Dogs return to Tsujido, despite warnings from Saejima's friends for him to stop hanging out with the Oni-baku, he remains loyal to the Oni-baku & insists on helping them out, despite Eikichi telling him not to get involved. Although he gets beaten down by Misaki & the rest of Saejima's friends, he is able to escape to inform Eikichi of Saejima & Kamata's plans. He & Itou Yui try to discourage Eikichi from facing Saejima alone but without success. He later comes to Ryuji's aid with Itou & Abe during Ryuji's fight with Kamata & stops Kamata's friends from ganging up on Ryuji.
Sometime later, Ryuji finds Itou's photo in Tsukai's wallet & realises that Tsukai has a crush on Itou. He & Eikichi plan a party to set up Itou with Tsukai, while allowing them access to Itou's female friends at the same time. Tsukai takes Itou shopping to prepare for the party & she reveals to him that she has a crush on Eikichi. Although heartbroken, Tsukai promises Itou that he will do whatever it takes to support her.
At the party, Tsukai tries to divert Eikichi's attention to Itou but Eikichi, along with the other guys are distracted by the new girl, Kaoru. Tsukai tries to cheer up a heartbroken Itou. He defends Itou from two guys from the Oni-baku's previous school & refuses to use the Oni-baku's name to win. Upon seeing Eikichi with Kaoru, he reveals to Eikichi that Itou has a crush on him & expresses his disappointment in Eikichi.
Tsukai challenges Eikichi to a fight for hurting the girl that he loves & puts up a fairly good fight before finally getting knocked down. Itou rushes towards the fight scene after being informed by Ryuji & tends to Tsukai's wounds. She reveals to Tsukai that this whole ordeal was orchestrated by Eikichi & Ryuji. Finally understanding his true feelings, Itou thanks Tsukai for all that he has done. They begin dating soon afterwards.
He later appears in numerous fights as the Onibaku's backup. During the Fumiya arc, he fights alongside Abe & Makoto against the police to allow Ryuji a chance to escape. During the Okubo arc, he offers Eikichi money to pay for Okubo's extortion demand & arrives as backup for Eikichi during his fight with Okubo. He is shown to greatly respect the Oni-baku, especially Eikichi & witnesses Eikichi cry over attacking Ryuji to prevent him from involving with Okubo.
He is very protective of Itou & defends her shady reputation from Hattori, his friend Kikumaru's senpai & a member of the Yamiko-Tei Gang. He attacks Hattori without hesitation & nobly informs Hattori to come to Tsujido if he ever wants to fight as he won't try to run. He realises that he doesn't care about Itou's past because he loves her.
Tsukai later arrives as backup & fends off Mami's father's associates, allowing Makoto & Mami a chance to escape. He tries to warn the Oni-baku about class 1-F's reputation when the Oni-baku repeated a year & is present to watch Minamino Youkou fight Ryuji. During the Idaten arc, he arrives as backup during Eikichi's fight with Takezawa. He is also unfortunate enough to be with Saejima & Kamata to witness Kiyoma's power.
During the Midnight Angel arc, he reveals to everyone Midnight Angel's past. He is very observant & is often the 1st to inform Eikichi news. He provides the Oni-baku with backup & gets thrown against the Kamakura Massacre Unit as a trap by Akutsu. When the Midnight Angel arc finishes, he takes part in the Oni-baku Gumi's celebrations and helps Ryuji & Nagisa paint their bus.
During Eikichi's fight with Natsu, Tsukai advises Eikichi to be careful & is the 1st to suggest to everyone else to stop the fight when it gets out of control. He is the 1st to inform the Oni-baku about the Yokohama invasion & rounds up the K. T. Butai & the Idaten gang as backup for the Oni-baku when they fight the Yokohama Calvary.
When Saejima introduced his new partner, Kiwamezawa Usagi, to the Oni-baku Gumi and challenges Eikichi to a fight, Eikichi nominates Tsukai as his partner in absence of Ryuji. However, Tsukai gets taken down easily by Kiwamezawa. Tsukai & Eikichi later appear to be defeated by Sakaki Ashura but it turns out that Sakaki was defeated by Eikichi.
During the Yagyo Atsuki arc, he helps the Oni-baku Gumi determine Yagyo Atsuki’s true identity through his contacts at Enoshima. He gets tricked by Tamaru & rushes to the gym to stop Eikichi & Nakajo from fighting, only to be ambushed & brutally attacked by Yagyo Atsuki.
During the feud between Eikichi & Ryuji, he refuses to take sides & tries to patch things up between the two, such as paying for the group’s restaurant bill & organising a BBQ. Despite his best efforts, he is ignored by his friends & confronts the root of the problem, Katsuyuki & Ken. When they refuse to cooperate, he takes them both on. However, at this time, Eikichi & Ryuji have already made up. It is during this time that Tsukai develops stomach cramps & hematuria due to stress.