Sakamoto-san, a black cat inhabiting Shinonome Laboratory, was originally named Taisho by his first owner, Kana Nakamura, but adopted his current identity from a box label after being discovered by Hakase and Nano. A red scarf crafted by Hakase grants him human speech by translating his thoughts. At 20 cat years old, he demands the honorific "-san" to assert his seniority over the lab’s inhabitants.

Previously under Kana’s neglectful care—enduring meals of instant ramen and painfully hot baths—he fled to Hakase and Nano, opting to remain with them despite their clumsy pet ownership. His aloof, authoritative demeanor often clashes with clandestine feline tendencies, such as chasing insects or batting at toys, which he hastily masks to uphold his dignified façade.

Hakase provokes him with pranks, dismisses his preferred title, and shifts blame onto him, yet their dynamic occasionally softens into collaboration, like jointly customizing a Daruma doll. Nano attends to his needs respectfully, though he still asserts dominance over her. He pointedly ignores admirers like Yuuko Aioi, indifferent to their curiosity about his speech.

He tolerates Hakase’s chaotic antics, including elaborate traps, while striving to appear unflappable. A temporary return to Kana reignited past trauma, cementing his loyalty to the lab. Across narratives, he navigates dual roles: a stealthily playful cat and a stern, self-appointed guardian clinging to authority.