Kenji is a high school delinquent who hangs out with friends Ota and Asakura, engaging in intimidation and minor confrontations, though no physical violence occurs on screen. His natural intimidation makes rival gang members retreat out of fear without direct interaction. His angular, minimalist design and often expressionless face create a deadpan comedic effect during extended pauses before he delivers understated yet impactful dialogue.

After accidentally receiving a bass guitar during a street robbery intervention, Kenji impulsively forms a band with Ota and Asakura despite having no musical experience, knowledge, or instruments. They take additional instruments from their school's music room, resulting in an unconventional lineup of two basses and minimal percussion. At their first practice, Kenji shows rare visible astonishment alongside his bandmates upon discovering the visceral satisfaction of creating sound, leading him to declare they should now be called "musicians." The band adopts the name "Kobujutsu" (Ancient Martial Arts) at Asakura's suggestion, which Kenji accepts without thought.

Kenji demonstrates unexpected appreciation upon encountering the folk band "Kobijitsu" (Ancient Fine Arts). He listens to their performance without confrontation and reciprocates by having them experience his band's abrasive, rhythm-driven noise music. When Kobijitsu's leader Morita invites Kobujutsu to a summer music festival, Kenji agrees despite having no repertoire or conventional skills. His initial enthusiasm wanes, leading to disengagement and a declaration of boredom with the band, resulting in a confrontation with classmate Aya. His bandmates remain confident he will participate in the festival.

On festival day, Kenji is intercepted by rival gang leader Oba. Instead of fighting, he unexpectedly smashes his bass guitar, produces a recorder, plays it while fleeing, and initiates a chaotic pursuit to the festival grounds. Arriving during Kobijitsu's malfunctioning set, Kenji rejoins Ota and Asakura on stage. During Kobujutsu's performance, his typical stoicism fractures, revealing intense passion as he sings out to the audience. The performance culminates in an improvised collaboration with Morita's band. Afterward, Kenji reverts to emotional reserve, matter-of-factly stating the band has disbanded while unexpectedly inviting Aya to visit Disneyland, indicating a subtle shift in their relationship. His bandmates observe this and anticipate his eventual agreement to reunite the band later that day.