Sanpei Mihira is a boy typically depicted aged 10 to 15, residing with his grandfather Ippei Mihira in a rural mountain village in Yokote City, Akita Prefecture. He speaks in a distinctive Tōhoku dialect. Sanpei is recognized by his consistent attire: a straw hat, white T-shirt layered over red long sleeves, blue pants, and traditional Japanese zōri sandals. His personality blends cheerfulness and optimism with strong moral principles, particularly concerning fair play and environmental respect during fishing. He exhibits intense focus and analytical problem-solving skills when angling, often leading opponents to underestimate him before recognizing his expertise; his eyes glow with heated intensity during competitions or challenging fishing scenarios.

Significant familial absence marks Sanpei's background. His mother died in his early childhood, and his father subsequently left him with Ippei before disappearing without further contact, leaving Sanpei with minimal memories of his parents. Throughout the series, Gyoshin Ayukawa travels with Sanpei while secretly investigating Sanpei's father's whereabouts. Unbeknownst to Sanpei, some rivals he faces in fishing competitions are connected to his father's past. Ippei serves as Sanpei's primary mentor, imparting extensive knowledge of fishing techniques, rod craftsmanship, and ethical engagement with nature. Despite his youth and initial inexperience, Sanpei develops into a highly skilled angler, eventually surpassing Ippei's abilities and earning recognition from seasoned professionals. His growth unfolds through numerous fishing contests, encounters with diverse rivals, and journeys across various natural landscapes in Japan and internationally, refining his technical abilities and deepening his philosophical understanding of fishing.

Key relationships define his journey: his foundational bond with his grandfather Ippei, who mentors him until Ippei's eventual passing, depicted in the manga's conclusion as found deceased while holding a fishing rod. Sanpei shares a childhood friendship with neighbor Yuri Takayama, characterized by frequent bickering but underlying mutual affection. His dynamic with the enigmatic Gyoshin involves mentorship and shared expeditions, including a significant journey in the live-action film adaptation to the fabled fishing ground Yonakidani. This film further explores Sanpei's family background, revealing his sister Aiko's attempt to relocate him to Tokyo for formal education, creating tension between familial expectations and his dedication to fishing. The narrative culminates in Sanpei's pursuit of a legendary giant Japanese char in Yonakidani, symbolizing a personal challenge and a connection to his missing father, whom Ippei hoped might return to confront the same fish.

Sanpei's character extends beyond the core series through licensed merchandise like fishing lures featuring his likeness. His portrayal remains consistent across media, emphasizing his unwavering passion for angling, resilience in overcoming challenges, and respect for the natural world.