Gyoshin Ayukawa is a mysterious and highly skilled fisherman who serves as a mentor to the protagonist, Sanpei. He first appears early in the story and becomes a recurring figure, recognized for his distinctive appearance and enigmatic past. He has black hair which he always wears in a ponytail, along with sideburns and semi-long whiskers. He is never seen without his sunglasses and a red hat, typically dressed in a red shirt, a fishing vest, and blue jeans. The constant sunglasses conceal a scar over his right eye, which he lost in a childhood accident. The accident occurred while he was fishing with his father, who, in a fit of drunken frustration after a poor day of fishing, swung his rod back and hooked his son in the eye. This traumatic event is the reason Gyoshin wears sunglasses and also why he chose to leave home at an early age, believing his absence would prevent his parents from worrying about his blindness and spare his father from the painful memory of the incident.

Despite his somewhat intimidating or rough appearance, Gyoshin comes from a very wealthy and educated background. He is a college graduate and an extremely qualified lawyer, and he possesses such athletic ability that he could have competed in fencing at the Olympic level. However, his primary obsession is fishing. He has traveled to countless fishing locations throughout Japan and around the world, driven by a deep passion for the sport and its challenges. His personality is that of a wanderer, a "furaibo" or masterless angler, who appears in different places to test his skills against legendary fish. In his early manga appearances, he was often depicted drinking whiskey, but the author received so many fan letters from high school girls that he gradually changed the character into a more refined and gentlemanly figure after the tenth volume of the manga.

Within the story, Gyoshin acts as a powerful rival and later a close ally and teacher to Sanpei. He recognizes the boy's natural talent and shares his extensive knowledge of fishing techniques, often taking him on expeditions to catch rare and giant fish. Beyond teaching him about angling, Gyoshin also undertakes a secret, personal mission on Sanpei's behalf: investigating the whereabouts of Sanpei's long-missing father. This quest adds a layer of depth to his character, transforming him from a simple fishing expert into a guardian figure who helps Sanpei navigate both the natural world and his own family history. He is the one who informs Sanpei about the legendary giant fish in the hidden valley of Yonakidani, which becomes a central goal for the young fisherman.

Gyoshin's notable abilities extend far beyond fishing. His precision and skill with a fishing rod are often likened to a form of martial art, reflecting his athletic prowess, particularly in fencing. His sharp, analytical mind, which qualifies him for a career in law, also makes him a brilliant strategist when it comes to outsmarting fish. He is versatile and intelligent, applying these traits to overcome the most difficult angling challenges. His physical scar and the tragic story behind it serve as a constant reminder that his journey is not just about sport, but also about a personal search for peace and meaning, turning him into one of the most complex and respected characters in the series.