Kimura, most often referred to as Kimura-sensei, is the classical Japanese literature teacher at the high school the main characters attend. He is a man of ambiguous age, typically appearing in a suit, with a pale, gaunt face and glasses that constantly reflect light, making his eyes impossible to see. One of his most prominent and unsettling physical features is his mouth, which hangs open in a slack-jawed expression for nearly the entire series. This was not always his appearance; in his first manga appearance, he looked entirely normal until he was asked why he became a teacher, at which point he seemed to suffer a kind of mental break and permanently adopted his now-familiar, eerie look.

His personality is defined by a singular, openly admitted, and all-consuming motivation. When asked directly by a student why he chose his profession, he famously and passionately declared it was because he likes high school girls. This statement is the key to understanding his role in the story. He is constantly finding excuses to be near the female students, such as lingering to observe swimming classes or physical examinations where students are weighed and measured. His comments are rife with inappropriate innuendo; on one occasion, he requested to be served the water from the swimming pool the girls had been using, and on another, he told a student he liked her gym shorts. His behavior is a source of intense discomfort and loathing for almost the entire female student body as well as some of the female faculty members.

Of all the students, Kimura has a very specific and unwelcome fixation on a girl named Kaori, whom he calls by the cutesy nickname Kaorin. He went so far as to have her transferred into his homeroom class for her final year, and he once asked her to call him by the equally infantilizing name Kimurin. This obsession peaks at the end of the school year when he chillingly announces to the entire class that he and Kaorin will be together forever. His interactions with other teachers also reflect his character; he once asked the teachers Yukari and Nyamo out for drinks, but they turned him down, finding him too creepy. Despite his revolting behavior, he is inexplicably popular with many of the male students, who admire what they perceive as his sincerity.

One of the most shocking and defining contradictions of Kimura's character is his family life. Despite his lecherous obsession with schoolgirls, he has a beautiful, kind, and devoted wife and an intelligent-looking daughter. When the main characters first see a photo of his wife, they refuse to believe it, to which Kimura proudly responds in heavily accented English, Mai waifu. His wife, bafflingly, finds him handsome and cool, a sentiment that leaves the students in a state of utter confusion. Furthermore, away from the school environment, Kimura is depicted as a responsible and kind-hearted citizen. He is seen donating generously to charities, picking up litter in the park, recycling cans, and even praying for world peace at a shrine.

Kimura has no notable abilities in a traditional sense, but his character is marked by several peculiar quirks. For unknown reasons, he often blurts out random phrases in English, despite not being an English teacher. His continued employment at the school, given his blatant and persistent sexual harassment of minors and colleagues, is one of the series' greatest and most absurd mysteries. In terms of development, Kimura remains largely a static character, serving as a recurring comedic antagonist. However, glimpses of his charitable, kind-hearted nature and his loving family provide a layer of complexity, suggesting that his lecherous school persona is only one facet of a deeply contradictory individual.