Itsuki Keizo is a professor at an agricultural university, where he serves as the head of a laboratory specializing in microorganisms and fermentation. He is an old acquaintance of Tadayasu Sawaki's grandfather, which establishes the connection that brings the protagonist to his laboratory. His exact age is never provided, but it is revealed that he served in the Japanese army during World War II, where his research interests allegedly included making aviation fuel from potatoes.

Personality-wise, Itsuki is portrayed as an eccentric and geeky academic, famously known on campus for his obsessive enthusiasm about his field. He has an endless capacity to discuss microbes and fermentation, a tendency that often causes those around him to flee to avoid a lengthy lecture. He possesses a distinct visual design, being drawn in a much more simplistic and cartoony style compared to most other characters in the series, which marks him as visually distinct. A man of eclectic tastes, his interests extend from fermented foods and alcoholic beverages like beer, Japanese sake, and wine to more unusual and disgusting delicacies from around the world, including chibiak, raw skate, and even the production of black powder.

Itsuki is a firm believer in practical learning, which he calls "Silhak" or practical science. This philosophy is central to his role as an educator and researcher. He values hands-on experience and experimentation over theoretical perfection or formal grades. For instance, he welcomes students who attempted to illegally brew sake for profit into his laboratory, appreciating their spirit of experimentation rather than punishing them. His research space famously includes a brewery installed in the basement, where he and his students actively produce a wide array of fermented goods like sake, miso, soy sauce, and vinegar. This commitment to practicality also extends to his views on food science, as he holds a realistic and positive opinion of chemical seasonings, emphasizing that the ability to choose ingredients is more important than declaring one inherently superior to another.

Within the story, Itsuki serves as the mentor and enigmatic leader of the laboratory. His primary motivation is his deep interest in microorganisms, a passion that reaches its peak with Sawaki's unique ability to see microbes with the naked eye. He is intensely curious about this power and views Sawaki as a crucial key to his research, which has grand ambitions such as terraforming other planets using microbes. Despite an early tendency to treat Sawaki almost as a subject of study, Itsuki ultimately proves to be a great educator who provides his students with a unique and valuable learning environment. He maintains a long-standing personal relationship with Haruka Hasegawa, having known her since she was a child; in a memorable and mischievous demonstration of his influence, he showed her his experimental facilities, which he refers to as a "chalet," a factor that led her to pursue graduate studies under him. He also has connections with influential figures in politics and business.

Itsuki shows a degree of character development when, through Sawaki's power, he is finally able to see and hear the very microorganisms he has studied for so long. The experience is so profoundly moving that it brings the normally stoic professor to tears. His notable abilities are rooted in his vast, encyclopedic knowledge of agricultural science and fermentation. He can identify and solve complex microbial problems, such as using cinder ash to neutralize a sake contamination that was spoiling a batch, demonstrating a deep understanding of historical and practical techniques. He is also known to have performed unorthodox procedures, such as using botulinum toxin for a minor plastic surgery on a female associate, further underscoring his unique and unconventional nature.