Kazuki Joushima serves as the homeroom teacher for class 2-2 at the high school where the story takes place, and he is also the school's music instructor. He is described as having a slender build and a long face, with a personality that is quite casual, cheerful, and open. Because of his approachable nature, his students affectionately refer to him by the nickname Shimaccho.

His primary role in the narrative is as a mentor and facilitator. Noticing a lack of volunteers for the upcoming community interaction exchange meeting, he takes the initiative to assign members for the committee, forcibly designating several of his students, including the main protagonist Jun Naruse, to the task. He is the first person to propose that the class perform a musical for the exchange meeting, a decision that forms the central project of the film. His personal music preparation room is famously cluttered and overflowing with his own belongings, a small detail that adds to his character.

Kazuki's motivations are rooted in a desire to help his students overcome their personal struggles. His methods are subtle but efficient, demonstrating a form of cunning mentorship aimed at pushing his most troubled students toward growth and self-awareness. By forcing a group of students with their own issues—which includes Jun, along with Takumi Sakagami, Daiki Tasaki, and Natsuki Nito—to work together, he creates the necessary conditions for them to confront their problems and develop meaningful relationships. His role is that of an instigator whose actions, while seemingly forceful, are intended to provide the students with an opportunity to communicate and heal. Information on his background before the events of the story or his notable abilities outside of his teaching and musical knowledge is not extensively documented.