Yoko gave birth to Kouta when she was 18 years old. When the father found out she was pregnant, he disappeared, and Yoko, defying the objections of those around her, gave birth alone and raised Kouta by herself. Being close in age, the two got along well and seemed more like friends than mother and son. One day, Kouta invited Yoko on a hot spring trip. Kouta had something he wanted to tell Yoko once he became an adult, and that's why he invited her on the trip.