Kankichi Ryōtsu, universally known by his nickname Ryo-san, is the central character of his story, serving as the chief patrol officer at a small police box in the Katsushika district of Tokyo. Physically, he is a middle-aged man with a short, robust, and sturdy build. He has a distinct crew-cut hairstyle, thick body hair, and visible stubble, but his most famous and recognizable feature is his thick, curving, M-shaped unibrow. While on duty, he wears the standard blue police uniform but distinguishes himself by keeping his sleeves permanently rolled up to his forearms and, most notably, forgoing regulation shoes in favor of wooden geta sandals, which he is still able to run in at surprisingly fast speeds. When off-duty, his attire is typically casual, consisting of shirts and long pants, or more relaxed clothing like open shirts and shorts during hot weather or holidays.

Ryōtsu's personality is a complex mix of laziness, greed, and unexpected competence. He displays a chronic lack of interest in his routine patrol or office duties, often preferring to spend his time on the job sleeping, building plastic models (a hobby he is deeply devoted to), or playing with video games and toys. His laziness, however, is matched only by his immense fervor and cleverness when it comes to devising get-rich-quick schemes. His greatest desire is money, a fixation rooted in his financially strained upbringing in the Asakusa district of Tokyo. To this end, he relentlessly invests in potentially profitable fads, scams, and business ideas, though these ventures almost always end in spectacular failure, causing significant property damage or great embarrassment to the police force and leaving him as penniless as before. Despite this, he possesses a surprising amount of street-smart resourcefulness and has an encyclopedic knowledge of Japanese culture, traditional games like shogi and kendo, and the latest trends in consumer technology and pop culture, allowing him to engage in deep discussions with his more affluent and worldly colleagues.

His role in the story is as the primary instigator of chaos and conflict, but also an unlikely hero. He lives a bachelor's life in the Niconico Hostel, a modest apartment building, and commutes to work on a creaky old white bicycle named Chidori, which he often rides late. His relationships are central to the narrative. The most significant is his volatile, paternal dynamic with his superior, Daijiro Ohara, whom he calls Bucho. Ohara constantly scolds and reprimands Ryōtsu for his laziness and harebrained ideas, but beneath the fury lies a genuine, fatherly care for his subordinate's well-being and a quiet pride in his rare successes. Among his colleagues, he shares a good-natured friendship with the wealthy Keiichi Nakagawa, who often finances his schemes while providing a global perspective that contrasts with Ryōtsu's local viewpoint. He also works alongside Reiko Katherine Akimoto, who often offers a female perspective and multilingual skills, as Ryōtsu speaks only Japanese and has little interest in learning other languages. A younger officer, Hayato Honda, often acts as a loyal but reluctant partner, getting dragged into Ryōtsu's misadventures. His family relationships are similarly complicated, marked by constant fighting with his hot-tempered father, Ginji, for whom he shares many traits, and a mixture of resentment and affection for his more successful younger brother, Kinjiro.

While he appears undisciplined, Ryōtsu possesses notable abilities. He has superhuman levels of strength, stamina, and tenacity, which he is forced to rely on in emergencies when he must capture criminals or resolve a crisis he helped create. He shows little interest in romance, though he occasionally attracts the attention of others, which never ends successfully. There is little development in the traditional sense, as he remains consistently penniless and prone to causing chaos. However, his character shows a reliable pattern of growth through failure, occasionally demonstrating sharp crime-solving instincts or a hidden dedication to his community, which reinforces his role as a flawed but indispensable and unexpectedly resilient fixture of his world. One recurring motif is the tally of financial damages caused by his antics, which underscores his chaotic essence. He also has a notable aversion to his birthday, March 3rd, which is also the Girls' Festival, a day that he believes is cursed and inevitably brings him terrible misfortune.