Dr. Haruka Nishikikōji serves as the elementary school doctor, operating from a rooftop clinic resembling a medieval castle. His physical build is notably overweight, specifically measured at 105 kg during an incident involving his saliva-detecting invention. He demonstrates strong commitment to assisting students with their problems through unconventional methods, frequently creating unique inventions to address health issues or school challenges. These inventions, though innovative, consistently result in comedic mishaps causing embarrassment for himself, school nurse Miyuki Koizumi, students, and faculty—exemplified by the saliva detector unintentionally revealing personal information instead of identifying physical traits.

Personally, he harbors longstanding romantic feelings for Miyuki Koizumi, the school nurse and his childhood friend. Persistent shyness and recurring interference from his invention mishaps prevent him from openly expressing his affection. His attempts to impress Miyuki by showcasing his inventions consistently backfire, further complicating his romantic pursuits.

His relationships extend to Miyuki’s younger sister Mayumi Koizumi, a student at the school, and other students including Yukihiro Hongō (who harbors a crush on Mayumi), Kaori Tajima (with a crush on classmate Gen), and Gen Genda. Faculty dynamics involve mixed interactions: Vice Principal Rokoro Shibuya and teacher Hajime Mizukoshi frequently scheme to dismiss him due to invention-related disruptions, while others like the muscular teacher Tamotsu Abe (who also pursues Miyuki) and easily startled teacher Hideko Ikeda directly experience the consequences of his experiments. He additionally created entities called "hungry germs"—Tai, Shoku, and Kin.

His professional methods, recurring invention failures, and unresolved romantic pursuit of Miyuki show no significant evolution or resolution across the series. His role remains consistent as a dedicated yet accident-prone figure driving comedic scenarios within the school.