Kiyomasa Ooiwa, husband to Setsu and adoptive uncle to Anna Sasaki through Setsu's kinship with Anna's foster mother Yoriko, resides with his wife in a rural seaside town within Hokkaido's Kushiro wetlands. He possesses grey or dark hair, black eyes, and a leaner build than Setsu. As a craftsman by trade, Kiyomasa displays pragmatic sensibilities and grounded advice—cautioning Anna against unsafe areas like the abandoned silo—while wielding a dry wit, joking about the Marsh House's hauntings. His demeanor reflects diligence, reliability, and a no-nonsense approach to duties. During Anna's asthma-related summer stay, Kiyomasa and Setsu provide temporary guardianship marked by warmth and freedom, balancing concern for her well-being with space for solitude and exploration. He actively supports Anna during conflicts, backing Setsu's defense against neighbors' criticism and aiding her at the Tanabata festival altercation. Kiyomasa further aids Anna's integration by introducing her to community figures like the fisherman Toichi, who assists her marsh travels. Though uninvolved in supernatural events, Kiyomasa’s steadfast presence and acceptance anchor Anna’s emotional stability. He and Setsu remain consistent pillars through her self-discovery, contributing to her improved emotional resolve by the journey’s end.