Gai Hidō is a school delinquent and third-generation member of the Hidou Boryokudan Group yakuza family. He is characterized by his ever-present, open-mouthed evil grin. Within the school setting, Gai harbors a romantic interest in Yukie Kokuhō, Kenichi's younger sister. His established role remains strictly that of a classmate and delinquent figure; his portrayal shows no significant development, expansion of background, elaboration on personal history, or changes in relationships or status across available media.