Junichirō Uchida is a supporting character in the romantic drama. He is introduced as the younger brother of Sara Uchida, who works as a lady's maid. Junichirō is a newly minted, low-level runner for the Aoba gang, a position that places him on the lower rungs of the yakuza hierarchy in 1923 Tokyo.

His personality is defined by a combination of youthful bravado and a genuine desire to protect his sister. Despite his association with the criminal underworld, his actions reveal a more impulsive and earnest nature. This is demonstrated when he sees Sara and her employer, Akiko Hashou, being harassed by a group of thugs. Without hesitation, Junichirō attempts to intervene and help the two women, showcasing his loyalty and protective instincts.

However, his role in the story is that of an inexperienced and outmatched fighter. His attempt to rescue Sara and Akiko fails almost immediately, as he is quickly and brutally beaten into a bloody mess by the thugs. This moment underscores his lack of power and influence within the gang, as well as the dangerous reality of the world he has chosen to inhabit.

His key relationship is with his sister, Sara, for whom he feels a strong sense of responsibility. He is also connected to the Aoba gang, specifically to the more senior and charismatic yakuza, Takao Itou, whom he regards as an older brother figure. It is Takao who reluctantly steps in to finish the fight and save Junichirō from a severe beating. Junichirō’s actions, though unsuccessful, serve as a catalyst for the initial meeting between Akiko, Sara, and Takao, setting the primary romantic tensions of the narrative into motion. Information regarding his personal growth, development over the course of the story, or any specific notable abilities beyond his role as a gang runner is not available.