Yuuji Shiina serves as a rival to protagonist Eisaku Yazawa ("Ei-chan"), aligning himself with Naoki Aizawa, the school's top delinquent, as part of Aizawa's group. He sports a skinhead look and positions himself as Ei-chan's direct adversary. Despite Ei-chan's physical weaknesses, Yuuji finds himself challenged by the protagonist's resourcefulness and unpredictable luck within conflicts among multiple factions.

Yuuji has an older sibling, Yuu Shiina, initially introduced as a "super ultra beauty" who enters a romantic relationship with Ei-chan. This relationship complicates the interpersonal dynamics when Yuu is revealed to be Yuuji's older brother. Yuu presents with traditionally feminine traits including long blonde hair, lipstick, and a chest-length hairstyle, though officially identified as male.