Kiyomasa Ōiwa lives with his wife Setsu in a rural seaside town between Kushiro and Nemuro, working as a carpenter who crafts wooden owls in his spare time. They have at least two children, including an adult daughter who works as a yoga instructor in Tokyo. Each summer, Kiyomasa and Setsu become temporary guardians for their adoptive niece, Anna Sasaki; Anna's foster mother, Yoriko Sasaki, is Kiyomasa's sister-in-law. He is talkative and caring, often expressing positive views about others in the community, yet maintains a notable aversion to Nobuko Kadoya's mother. He refuses to deliver his carpentry work to her personally, jokingly claiming an "allergy" to her. While initially relaxed about Anna's emotional struggles, he remains composed when situations become serious. Kiyomasa lightly teases Anna about local folklore, suggesting the abandoned silo and Marsh House are haunted by ghosts – unaware these locations are significant to Anna's experiences with Marnie. He advises her to avoid these sites but encourages her to accept boat rides from the local fisherman, Toichi. When Anna suffers a severe fever, he helps carry her indoors and later facilitates her reunion with Yoriko. His presence contributes to the supportive environment around Anna. His name, Kiyomasa, combines Japanese characters meaning "clear, pure, clean" and "right, proper," while his surname Ōiwa means "big rock." The character draws inspiration from Mr. Peggotty in Joan G. Robinson's original novel *When Marnie Was There*.