Hamako Shihonmatsu works as an animation director at Studio Petit. Her chain-smoking habit is central to her initial portrayal. An attempt to quit smoking during the studio's concurrent production of three anime series spirals into a nervous breakdown. This manifests in episodes where she unconsciously tries to light various objects as cigarette substitutes, highlighting the intensity of her nicotine withdrawal and stress.

She maintains a professional relationship with Mikiko "Kuromi" Oguro, assisting her with production management. Hamako's experience makes her a champion of quality against the new producer, Takashimadaira, whose focus on deadlines undermines animation standards. Their conflict over artistic integrity versus scheduling pressures erupts in confrontations about declining work quality, culminating in Hamako angrily leaving the studio. Her departure forces a critical decision about the studio's direction.

Her personality blends cynicism and dedication. Early portrayals depict her as cold, callous, and snarky, often delivering blunt advice like a flat-toned "Don't die" to Kuromi. Despite this abrasive exterior, she demonstrates deep investment in the studio's output and forms a crucial professional partnership with Kuromi through their challenges. Her breakdown and walkout stem directly from her commitment to preserving animation quality against external pressures degrading the work.