Kiyomasa Ōiwa shares a home with his wife, Setsu Ōiwa, in the rural coastal town of Kissakibetsu within Hokkaido's Kushiro wetlands, serving as an uncle figure to the protagonist, Anna Sasaki. Their adult children live independently, including a daughter working as a yoga instructor in Tokyo and at least one son. Kiyomasa works as a carpenter, finding enjoyment in woodworking during his free time, particularly crafting owls. He possesses a talkative and caring nature, often speaking positively about others. Kiyomasa maintains a relaxed attitude towards Anna’s emotional struggles, reserving concern only for serious problems and responding calmly when they arise. He demonstrates fondness for Anna through frequent, light-hearted teasing, such as joking about the local abandoned silo and Marsh House being haunted—unaware these remarks hold deeper significance for her. His humor surfaces occasionally, like claiming an "allergy" to Mrs. Kadoya due to his strong dislike, leading him to delegate deliveries to her through Setsu. Within the household, Kiyomasa and Setsu share responsibilities, with Setsu and Anna handling cooking and harvesting tasks. His extended family includes Setsu’s sister, Yoriko Sasaki (Anna’s adoptive mother), as his sister-in-law. His given name "Kiyomasa" (清正) combines the characters for "clear/pure" (清) and "right/proper" (正), while his surname "Ōiwa" (大岩) means "big rock" (大 for big, 岩 for rock). He remains a consistently supportive and steady presence throughout Anna’s stay, without undergoing significant character development.