Chiyako Matsuura, originally known as Chiyako Koishikawa, is the mother of Yuu Matsuura and a central figure in the unconventional family situation that drives the story. Prior to her marriage, her maiden name was Namiki. Her life takes a dramatic turn when she and her husband, Youji Matsuura, meet Jin and Rumi Koishikawa on a trip to Hawaii. A romantic entanglement leads to the two couples deciding to divorce and swap partners, with Chiyako leaving Youji to marry Jin Koishikawa, Miki's father. This decision results in a complex living arrangement where the newly formed families share a home.

Chiyako is characterized by her warm and kind-hearted nature, approaching her new stepdaughter, Miki Koishikawa, with the gentle demeanor of a Cool Big Sis rather than a pushy parental figure, as she is mindful of not wanting to intrude on Miki's feelings. Unlike her son Yuu, who is initially more detached, Chiyako embraces the new family dynamic with optimism.

Her past holds significant emotional weight. Before marrying Youji, Chiyako had a relationship with Jin Koishikawa that resulted in a pregnancy. However, the child was tragically stillborn. A letter written by Chiyako's mother about this event later becomes a source of a major misunderstanding for Yuu. Furthermore, prior to her marriage to Youji, she was involved in an affair with the father of another character, Satoshi Miwa, while working at his firm. She ended that relationship and subsequently married Youji Matsuura, who was aware that she was already pregnant at the time of their wedding.

Throughout the series, Chiyako acts as a supportive and loving mother to Yuu. She is involved in his life, such as when she proposes he appear in a television commercial for the cosmetics company where she works. In the sequel, Marmalade Boy Little, she and Jin have a son named Hajime, solidifying their new family unit alongside Rumi and Youji's daughter, Rikka.