Chiharu Yaguchi is a key supporting character in the story, presented as an elderly woman living in Japan. Her outward identity is that of a modest watch repair shop owner, a seemingly quiet and unassuming profession that belies her true expertise.

Before retiring to a legitimate life, Chiharu was a highly skilled master gunsmith operating within the criminal underworld. Her talent was renowned for its precision, particularly her extraordinary ability to replicate original firearms with perfect accuracy, which made her a sought-after figure by various criminal elements. She has since left that world behind, but her reputation as an exceptional armorer persists.

Daisuke Jigen seeks Chiharu out specifically to repair his trusted Smith & Wesson Model 19 revolver, a weapon that holds immense personal value to him. Despite her retirement, she agrees to help him, demonstrating her continued skill. Her relationship with Jigen is initially professional, but it evolves into a partnership of mutual respect. She acts as a calm and pragmatic presence, contrasting with Jigen's more outwardly rugged demeanor.

Chiharu’s role in the story extends beyond that of a mere technician. She becomes a central figure in the narrative when a young girl named Oto comes into her shop seeking a weapon. This event draws both her and Jigen into a conflict with a powerful and ruthless crime syndicate. In the resolution of the story, her character is further developed when she makes the compassionate decision to adopt Oto after the girl's mother is killed, offering the orphan a stable and caring home. This action reveals a deep sense of responsibility and a protective nature, showing that her strength lies not only in her craftsmanship but also in her capacity for kindness. Her background as a former black market gunsmith is crucial to the plot, as her unique abilities are the catalyst for Jigen’s journey.