Onsen Mark is a middle-aged teacher at Tomobiki High School and a graduate of the same school. He serves as Ataru Moroboshi's homeroom teacher, teaches several subjects over the course of the series with English as his main subject, and also acts as the head of student discipline. His real name is rarely used; students, colleagues, and even the school principal call him Onsen Mark, a nickname taken from the hot spring symbol that decorates his clothing. The name itself means "mark of the hot springs," and in the original manga the symbol appears all over his suit and slacks, while in the anime it is limited to his tie and the back of his jacket.

Onsen Mark is earnest, serious-minded, and devoted to his work, but he is perpetually unsuccessful in carrying it out. He lives alone in a shabby boarding house and pours all of his energy into teaching and maintaining order, yet he has almost no control over his students. His lessons are constantly interrupted, and the classroom is frequently destroyed by the chaos caused by Lum, Ten, Shinobu, and the rest of the class. He often creates strict rules and disciplinary measures, which makes him unpopular with students and an easy target for Ataru, who delights in causing him trouble. Despite the endless failures, humiliation, and physical abuse he endures, he remains genuinely committed to educating his students and never gives up on them, even those who shock him, punch him, or throw explosives at him.

His central motivation is to be a proper teacher and to keep the school functioning, though his efforts almost never succeed. In the story he serves as the beleaguered authority figure, generating comedy through the contrast between his sincere attempts to enforce rules and the complete disorder surrounding him. His most important relationships are with his students. Ataru is his chief source of frustration, regularly derailing lessons and undermining his position. Lum is impossible to discipline through ordinary means because of her alien nature and unpredictable behavior, while Ten and Shinobu frequently contribute to the destruction of his classroom. His personal history as a graduate of Tomobiki High gives him a deeper attachment to the school than an ordinary staff member might have.

In one notable storyline, rumors spread that Onsen Mark plans to transfer to another school, and the rumors prove true. Before leaving, he asks the principal to organize a large school-wide race so that he can carry a good memory of his students with him, a moment that reveals his sincere affection for the pupils who make his professional life so difficult. Onsen Mark possesses no supernatural abilities or special combat skills; his defining traits are his patience, persistence, and unwavering sense of duty. His endurance is particularly striking, as he continues to arrive at class and attempt to teach no matter how many times the room is wrecked or his authority is ignored.