Setsu Oiwa, Yoriko Sasaki's older sister and Kiyomasa Oiwa's wife, lives with her husband in a rural seaside town nestled between Kushiro and Nemuro on Hokkaido. The couple raised at least one daughter and one son there before both children relocated to the city as adults. Their daughter became a yoga instructor in Tokyo, after which Setsu preserved her bedroom untouched, unable to emotionally renovate the space. Setsu displays a kind, patient, and laid-back demeanor. Unlike her sister Yoriko, she avoids excessive concern over behavioral issues—like Anna’s late returns or insults toward local girl Nobuko Kadoya—calmly advising apologies instead of punishment. Her relaxed approach to child-rearing reflects understanding of Anna’s emotional struggles. When Yoriko sought frequent parenting advice after adopting Anna, Setsu eventually requested fewer calls due to her own commitments. At Yoriko’s request, Setsu and Kiyomasa hosted Anna for the summer, hoping Hokkaido’s coastal air would ease her asthma. Though they had met Anna in early childhood, she did not recognize them upon reuniting. Setsu settled Anna into their daughter’s preserved bedroom and encouraged independent exploration of the town. She actively fostered social connections, particularly urging Anna to befriend Sayaka, a neighbor’s daughter of similar age. During the Tanabata Festival, Setsu lent Anna their daughter’s yukata. After Anna clashed with Nobuko and fled, Mrs. Kadoya confronted the Oiwas with exaggerated accusations. Setsu and Kiyomasa defended Anna, dismissing the claims as unfounded, and Setsu reassured her the conflict would resolve peacefully. She involved Anna in domestic tasks like harvesting tomatoes, expressing relief at her improved spirits and mentioning Yoriko’s persistent calls driven by maternal worry. Setsu shared photographs of Anna sent by Yoriko, reinforcing family ties. Her support extended to practical aspects of Anna’s emotional journey. Setsu provided a stable, non-judgmental environment where Anna processed her experiences with Marnie and gradually opened up to others. This consistent presence and unconditional acceptance indirectly aided Anna’s emotional growth and eventual reconciliation with her identity.