Setsu Oiwa is the wife of Kiyomasa Oiwa and the older sister of Yoriko Sasaki in When Marnie Was There. Setsu lives with her husband in a small, rural town where Yoriko's foster daughter Anna stays during the summer.
Setsu is a kind and sweet woman. Unlike Yoriko, Setsu is very laid back and patient when it comes to Anna's issues and doesn't get too concerned when Anna gets herself in trouble like coming home late or when Anna insults a resident girl, Setsu doesn't punish Anna but suggests that she apologizes.
Setsu became a mother to one daughter and at least one son whom she and her husband raised in their town until their children grew up and moved to the city, where their daughter became a yoga instructor in Tokyo. When her daughter left Setsu kept her bedroom the way she left it since it made her sad to renew it so the bedroom was left as if it was still occupied. When Yoriko and her husband adopted Anna, Yoriko sent her relatives some pictures of the new family together and she often called Setsu for parenting advice to the point where Setsu had to tell her to stop calling so often since she also had things to do, much to Yoriko's dismay. Setsu and Kiyomasa met Anna when she was much younger but she doesn't recognize them once they reunite years later.
The couple agree to have Anna live with them for the summer per Yoriko's request in hopes that Anna's asthma will get better. Setsu lets Anna sleep in her daughter's old bedroom, explore the town and encourages her to socialize with her neighbor's daughter who is around Anna's age.