Junko Yoshioka's father, a police officer, died under suspicious circumstances linked to corruption between Ōshika Construction and politician Seijiro Matsuzaka. This drives her to join Jotaro Zaizen's specialized team within the Government Crime Investigation Agency. Zaizen recruits individuals personally harmed by the corruption they target.

She embodies the civilian perspective, specifically the loss of family due to collusion between politicians and businesses. Her background fuels her personal stakes in the conflict against Keiichi Suzuki and the Haoukokuryu-kai yakuza group. Her character arc centers on seeking justice for her father while aiding Zaizen's investigations.

Despite her youth, Junko delivers speeches meant to inspire the team and bolster their resolve during confrontations. Her narrative blends vulnerability from her personal loss with determined resolve, contributing to the team's moral core. She is positioned as both a victim seeking closure and an active participant in dismantling corrupt networks.