Kirimaru Setsuno is a ten-year-old first-year student at the Ninjutsu Academy, recognized as one of the main trio alongside his classmates Rantarou and Shinbei. His name indicates he is from the Settsu region, as he has no surname of his own. This is because Kirimaru is a war orphan who lost his family and home when his village was burned during a conflict. This tragic background is the defining foundation of his character, forcing him to become entirely self-reliant from a very young age. Before he could even enroll in the academy, he survived by selling boxed lunches on battlefields and taking on any work he could find, saving every coin to pay his own tuition and living expenses.

This difficult childhood has forged in Kirimaru a personality that is both remarkably resourceful and intensely pragmatic. His most defining and memorable trait is his extreme, almost legendary, fondness for money. He is a self-proclaimed genius part-timer who constantly juggles numerous jobs, from childcare and pet handling to working at festivals and shops, in his relentless pursuit of income. This avarice manifests in comical but impressive ways; he can detect the sound of a coin dropping from several kilometers away, identify a coin's value by its sound or smell, and perform rapid eight-digit mental calculations, but only when money is involved. His eyes are said to physically transform into the shape of gold coins at the mere mention of treasure or profit. He is famously weak to enticing words like "free" or "cheap," and conversely, he hates the concepts of "loss," "payment," or doing something for nothing. In a game of rock-paper-scissors, he cannot make anything other than a fist because he never lets go of anything he has.

Despite this overwhelming stinginess, Kirimaru is fundamentally a bright, energetic, and stubborn boy. He is often the sharpest and most street-smart of his friend group, possessing a quick wit and a wealth of practical knowledge born from his struggles. His ninjutsu skills are also notably high, ranking among the top of his class, which suggests a natural talent that is often overshadowed by his preoccupation with work. He is a member of the Library Committee and diligently performs his duties, though this is largely motivated by his fear of the committee's imposing chairman. While he may be academically average or poor due to his focus on part-time jobs, his ability to think on his feet and apply practical solutions makes him a valuable asset during missions and school events.

Kirimaru's role in the story is that of the cynical, grounded foil to the optimistic Rantarou and the simple-minded Shinbei. Their friendship is strong, built on mutual support despite his constant schemes to profit from any situation. While he would generally demand payment for his help, when it comes to the safety or well-being of his two closest friends, he acts without a second thought, revealing that his relationships are more valuable to him than money. His most significant relationship is with his homeroom teacher, Hansuke Doi. Because Kirimaru has no family to return to, he stays at Doi-sensei's house during school vacations, and the two have formed a bond akin to that of a family. Both survivors of war, they share an unspoken understanding, and Doi serves as a guardian figure, even as he is often reluctantly dragged into Kirimaru's part-time jobs.

Throughout the series, Kirimaru shows a subtle but steady development. While his core personality and love for money remain unchanged, the narrative often presents situations where his deep-seated kindness and loyalty overcome his mercenary instincts. His harsh past is not forgotten but is presented as the source of his strength and independence. He also has a notable skill for cross-dressing, appearing as a girl named Kiri-ko for various missions or part-time jobs, a talent he performs with surprising naturalness and effectiveness. He has proven his popularity as a character, even ranking first in an official popularity poll for lower-class students. Kirimaru is a complex figure whose comical obsession with money is a direct and poignant result of a childhood spent fighting for survival, making him both a source of humor and a character of surprising depth.