Hiroshi Mado is the eccentric and often disheveled science teacher at the middle school around which the story takes place. A middle-aged man with unkempt white hair and a perpetual three-day stubble, he is almost always seen wearing a white lab coat, a look that firmly establishes his identity as a classic, if unconventional, researcher. His primary defining characteristic is that of a mad scientist, a man whose all-consuming passion for research and scientific curiosity consistently overrides common sense and ethical boundaries. He is blunt, sarcastic, and frequently refers to his students as little brats, showing little patience for the everyday duties of teaching, which he views as a necessary annoyance rather than a calling.

Despite his rough and often immature exterior, Hiroshi possesses a surprising sense of responsibility when it comes to his job, grudgingly helping students with remedial tests and providing academic support. His abrasive personality is balanced by a genuine, if odd, devotion to his family, particularly his wife Romi, whose important dates he never forgets. He has a childish and petty side, easily annoyed and prone to arguments, yet is unexpectedly reliable in times of crisis, making him an indispensable figure when supernatural chaos inevitably ensues.

Hiroshi was originally a researcher specializing in autonomous slimes, which are essentially gel robots. In a strangely mundane twist for a would-be mad scientist, he was expelled from his academic society not for forbidden experiments, but because he stubbornly refused to pay his membership fees. This expulsion is the source of an ongoing vendetta, and his entire motivation in the story is to use the living slime Puniru as the subject of groundbreaking research in order to regain his lost academic standing and seek revenge on the society that rejected him. Consequently, he sees teaching as a secondary role, a simple means to fund his true passion for research.

In the narrative, Hiroshi serves as a unique antagonist and a comic foil. While other characters have accepted Puniru as a cute girl, Hiroshi persists in referring to her solely as a slime organism, viewing her strictly as a scientific subject rather than a person. His approach to this research is questionable at best, involving experiments like attempting to dissect Puniru or push her off the school roof to test her limits. His key relationships are defined by his scientific obsessions. He has a contentious but trusting relationship with the student Kotaro, who often comes to him when problems arise. The student Mami Kirara becomes his unofficial research assistant, helping him in his work. He also creates an artificial slime lifeform named Gelee, whom he treats with a harsh, utilitarian mindset, willing to shut the AI down if it does not perform.

Despite his fixation on science, Hiroshi has a surprisingly warm home life. He is a doting husband to his laid-back wife Romi and the father of a mischievous son named Washihito, whose own eccentric interests mirror his father's. Over the course of the story, he shows a degree of growth, shifting from a purely self-serving researcher to a more collaborative figure, even as he remains largely unconvinced that artificial life deserves human-like empathy. His long-winded and complex scientific lectures about supernatural phenomena are a running gag, often ignored or fast-forwarded by the other characters, sometimes even by himself. In a surprising turn of events that highlights his popularity, Hiroshi won first place in a character popularity poll, surpassing the titular slime herself. His notable abilities are his keen scientific intuition and his technical expertise in robotics and artificial life, making him the only character who seriously attempts to unravel the mysteries of the world with a logical, scientific mind.