Setsu Ōiwa lives with her husband, Kiyomasa Ōiwa, in the small rural seaside town of Kissakibetsu, Hokkaido. Kind, patient, and laid-back, she demonstrates particular understanding towards her foster niece, Anna Sasaki, responding to challenging behavior like tardiness or insults (such as one directed at local girl Nobuko Kadoya) with advice to apologize rather than punishment. Setsu and Kiyomasa are parents to at least one son and one daughter. Their daughter moved to Tokyo to work as a yoga instructor. Setsu finds it emotionally difficult to alter her daughter's preserved bedroom, keeping it as a reminder. Before Anna's arrival, Setsu maintained regular contact with her sister, Yoriko Sasaki, offering parenting advice after Anna's adoption, though she later asked Yoriko to reduce call frequency due to her own commitments. Following a doctor's recommendation to improve Anna's asthma, Setsu and Kiyomasa host Anna for the summer. Setsu provides Anna with their daughter's former bedroom and encourages her to explore the town and socialize, especially with Nobuko. She facilitates Anna's participation in local activities, lending her daughter's yukata for the Tanabata festival. When Mrs. Kadoya confronts them about a conflict between Anna and Nobuko, Setsu and Kiyomasa defend Anna against exaggerated accusations and later reassure her they will resolve the situation. During interactions like harvesting tomatoes, Setsu expresses relief that Anna is enjoying her stay. She mentions Yoriko's persistent phone calls, reflecting Yoriko's ongoing concern, and offers to show Anna photographs sent by Yoriko, bridging the connection between Anna and her adoptive mother. Setsu remains a consistent, supportive, and stabilizing figure for Anna throughout the summer.