Yūichi Shiina operates as a prominent delinquent within the narrative, aligned with Naoki Aizawa's rival faction. This affiliation establishes him as a direct antagonist to the protagonist, Eisaku Yazawa ("Ei-chan"). Physically, Shiina adopts a skinhead appearance, reinforcing his delinquent identity and role in Aizawa's group. His interactions with Ei-chan consistently erupt into conflict, fueled by their opposing ambitions and group loyalties.

A pivotal plot development centers on Shiina's family: A super ultra beauty character forms a romantic bond with Ei-chan and is later revealed as Shiina's sibling. Initially introduced as Shiina's older sister, this character is unexpectedly disclosed to be his older brother, introducing a twist tied to gender identity or presentation. This revelation directly reshapes Shiina's dynamic with Ei-chan, infusing their rivalry with familial complexity.