Setsu Oiwa, wife of Kiyomasa Oiwa and older sister to Yoriko Sasaki, shares a home with her husband in a rural seaside town nestled between Kushiro and Nemuro on Hokkaido. They raised at least one son and one daughter there; their daughter now works as a yoga instructor in Tokyo. After the children left, Setsu found it too painful to alter her daughter's bedroom, leaving it untouched. Setsu maintains a close bond with Yoriko, offering parenting advice following Yoriko's adoption of Anna. Yoriko regularly sought guidance until Setsu requested fewer calls due to her own commitments. Setsu and Kiyomasa met Anna when she was young, though Anna doesn't recall them years later. When Anna's health falters, they agree to host her for the summer at Yoriko's urging, hoping the clean air will ease her asthma. Exhibiting a kind, patient, and laid-back nature, Setsu contrasts with Yoriko's more anxious approach. She grants Anna significant independence to explore the town and encourages friendships with local children like Nobuko Kadoya and later Sayaka. Setsu avoids reprimanding Anna for tardiness or her outburst at Nobuko, instead calmly suggesting an apology. She expresses sincere happiness seeing Anna enjoy her stay and soothes her after conflicts, such as Mrs. Kadoya's complaint about Nobuko's story. During Anna's visit, Setsu welcomes her participation in daily tasks like harvesting garden tomatoes. She shares family photos sent by Yoriko and mentions Yoriko's frequent calls expressing concern for Anna. This supportive environment fosters Anna's gradual emotional opening, reflecting Setsu's nurturing focus on providing stability without pressure.