Yoshiko Tsuyama belongs to protagonist Yosuke Eguchi's core group, attending the same high school as him and his biker gang members. Her significant character arc centers on an ongoing romantic relationship with Eguchi. This relationship evolves throughout the series against the backdrop of the gang's activities and high school milestones like school trips, cultural festivals, and Christmas celebrations.

Approaching graduation, Yoshiko Tsuyama and Eguchi's delicate relationship reaches a pivotal stage, facing unresolved aspects near a turning point just before the ceremony. This romantic storyline forms an emotional throughline alongside the gang's final adventures and farewells. The conclusion of their high school years brings focus to the resolution of their relationship, marking a significant element of the series' closing narrative.

Yoshiko Tsuyama actively interacts with other key characters, including Akira Ishikawa, Kakuji Marukawa, Yoshimi Harasawa, and Shinji Sakurai. She is integrated into the group's social sphere, contributing to the depiction of their personal lives beyond biker gang identities. The series frames her relationship with Eguchi as an important emotional component of his character development, particularly during their final year.