Haruka Nishikikōji is a school doctor of twenty-five years of age and serves as the director of the Haruka Clinic, a facility that doubles as the health center for Nanahoshi Elementary School. His clinic is noted for its unusual architectural design, resembling a medieval castle or a strange mansion situated on the school's rooftop. Physically, Haruka is considerably overweight. His weight is specifically recorded to be 105 kilograms, a figure obtained during an incident involving one of his own inventions.

Regarding his personality, Haruka possesses a notable dichotomy between his outward appearance and his inner nature. His face is often described as reminiscent of Frankenstein's monster, yet internally he is a very kind, timid, and generous individual. He is deeply dedicated to supporting his students and cannot bear to turn away any child who is facing trouble or needs assistance. Despite his profession and intelligence, he is also characterized by an eccentric and comedic disposition, frequently serving as the catalyst for the chaotic situations that unfold around him.

His central motivation is twofold. The primary driving force in his life is his longtime romantic affection for Miyuki Koizumi, the nurse at his clinic and his childhood friend. He is perpetually attempting to impress her and convey his true feelings, but his overwhelming timidity persistently prevents him from being forthright. In the source manga, they were described as fiancés, but this specific setting was altered for the anime adaptation, where his feelings are presented as an unspoken, one-sided love. His second major motivation is a genuine desire to help his students, which he pursues through his other defining trait: his passion for invention.

Haruka plays the central role in the story's narrative, acting as the primary engine for its comedic events. His role is to create and test numerous unique and often outlandish "dream inventions," such as a helmet meant to induce cheerfulness or lipstick that compels kisses. While his intentions are to solve problems and assist others, these creations are almost invariably failures. They malfunction in spectacular ways, leading to highly embarrassing, humorous, and chaotic situations that affect the students, the faculty, and Miyuki. After the resulting chaos, Miyuki typically resolves the incident, often by physically reprimanding him.

His key relationships define much of his interaction with the world. His most significant relationship is with Miyuki Koizumi, the object of his affections and the frequent recipient of his failed romantic gestures. Other important relationships include those with the students at the school, such as Mayumi Koizumi (Miyuki's younger sister) and Yukihiro Hongō. He also interacts with the faculty, including Vice Principal Rokoro Shibuya and teacher Hajime Mizukoshi, who often scheme to have him dismissed due to the disruptions he causes, and other teachers like Tamotsu Abe and Hideko Ikeda who are directly affected by his experiments. He is also credited with creating three entities called "hungry germs" named Tai, Shoku, and Kin.

In terms of development, Haruka's character shows little to no significant evolution across the series. His professional methods remain consistently accident-prone, his inventions never become more reliable, and his romantic pursuit of Miyuki reaches no clear resolution or deeper progression. He remains a static character, a kind-hearted but perpetually failing genius whose well-meaning but flawed schemes drive the episodic, repetitive nature of the comedy.

His notable abilities are ironically tied to his failures. He is presented as being very intelligent, possessing the knowledge and creativity of a skilled inventor and, by extension, a qualified doctor. However, his most defining "ability" is his capacity to create incredibly innovative devices that consistently and spectacularly backfire, turning his genius into a source of perpetual comedic disaster.