Hamako Shihonmatsu is a veteran animation director working at Studio Petit. Her career at the studio began as an animator before she rose to her more senior position, a trajectory that has granted her decades of hands-on experience in the high-pressure world of anime production. This extensive background has made her a high-class specialist and a cornerstone of the studio's creative output, though her initial demeanor often masks her true value to the team.

On the surface, Shihonmatsu presents a cold, sharp-tongued, and aloof exterior, carrying herself with a brusque, no-nonsense attitude. She is frequently seen with a cigarette in hand, a heavy smoker whose habit is a central part of her character design. A recurring personal struggle is her attempt to quit smoking, which she undertakes because she feels smoking women are "not pretty," though the stress of production often makes this difficult, sometimes leading to nervous episodes where she unconsciously tries to light other objects as substitutes. Despite her cynical and seemingly unapproving nature, those who work with her come to understand that she is deeply caring and warm-hearted beneath her gruff facade. She is known for delivering blunt, pragmatic advice in a flat monotone, such as a terse "Don't die," which, while abrupt, comes from a place of genuine concern for her colleagues' well-being.

Shihonmatsu's primary motivation is a fierce commitment to the integrity and quality of the animation her studio produces. She feels a profound sense of responsibility for the final product and adheres to a personal philosophy that it is better not to do the work at all than to do it poorly. This dedication puts her in a position of championing artistic standards against the systemic pressures of tight deadlines and quota-filling.

In the story, she serves as a crucial, though reluctant, mentor to the young production manager Mikiko "Kuromi" Oguro. While she may seem unapproving, she provides Kuromi with useful tips and strategies to navigate the chaotic workflow, imparting survival skills learned from her years in the trenches of animation. Her most significant role in the narrative occurs when Studio Petit, already struggling with a heavy workload, hires a new veteran producer named Takashimadaira. This producer prioritizes meeting deadlines at all costs, a method that severely compromises the quality of the animation. Shihonmatsu's conflict with Takashimadaira over artistic integrity versus scheduling pressures quickly escalates, leading to furious confrontations about the declining work standards. Her devotion to quality ultimately drives her to make the dramatic decision to angrily leave the studio, forcing Kuromi to confront the difficult choice between preserving the work's quality or simply making the deadline.

Shihonmatsu's key relationships are defined by her professional partnerships. Her dynamic with Kuromi evolves from one of a distant, critical superior to a more collaborative alliance based on mutual respect, as they face production challenges together. Her antagonistic relationship with the producer Takashimadaira serves as the central conflict of the sequel, representing the timeless struggle between the artistic soul of animation and the commercial realities of its production. While her characterization remains largely static, she demonstrates weathered adaptability, shifting her role in response to the changing managerial tides at the studio before her eventual departure. Shihonmatsu's notable abilities include her technical mastery of animation and her shrewd, almost intuitive understanding of other animators' personalities and motivations, which she subtly uses to boost productivity and safeguard the studio's output from behind the scenes.