Chiyako Matsuura, originally known by her maiden name Namiki, is the mother of Yuu Matsuura and a central figure in the family dynamics that drive the story. Her background is marked by a complex romantic history. During her university years, she was deeply in love with Jin Koishikawa, the man who would later become the father of Miki Koishikawa. A misunderstanding involving Miwa, the father of Satoshi Miwa, led Jin to believe Chiyako had been unfaithful, causing them to separate. At this time, Chiyako discovered she was pregnant with Jin's child, but he had already moved to London with Rumi. In her vulnerable state, her close friend Youji Matsuura proposed marriage to support her and raise the baby together. However, the physical and mental strain of the situation led to a stillbirth. Despite this tragedy, Youji remained committed, and the two married. A year later, they had a son, Yuu, who is Youji's biological child. Years afterward, during a coincidental family vacation in Hawaii, Chiyako reunited with Jin and Rumi. They resolved their past misunderstandings and, acknowledging that their original romantic feelings remained, decided to divorce their current spouses and swap partners, leading to Chiyako marrying Jin.

In terms of personality, Chiyako is portrayed as a warm and considerate woman. She is a skilled cook and works in the cosmetics industry. She is also mindful of her role as a stepmother; she deliberately avoids being overbearing or pushy with Miki, instead adopting a friendly, older-sister-like demeanor to build a comfortable relationship with her new stepdaughter. Her actions are consistently motivated by a desire for genuine emotional fulfillment and the happiness of those she loves, even if it means making unconventional life choices. The decision to swap partners, while shocking, stems from a belief in rekindling authentic love after years of prioritizing friendship and familial duty in her marriage to Youji.

Chiyako’s primary role in the story is as a catalyst for the main plot. Her decision with Youji to divorce and swap partners with Jin and Rumi creates the unique and confusing blended family situation that her children, Miki and Yuu, must navigate. Her key relationships are therefore central to the narrative. With her son Yuu, she shares a loving bond, though her choices directly impact his life. Her relationship with Miki is carefully cultivated, aiming for friendship over authoritarian parenting. Her relationship with Youji evolves from a supportive friendship into a marriage of comfort and mutual care, which eventually transforms back into friendship so they can pursue their first loves. Her most significant relationship is with Jin Koishikawa, the man she loved in her youth and ultimately reunites with.

Throughout the series, Chiyako undergoes notable development, moving from a woman who made a practical decision to marry a good friend after a devastating loss to someone willing to upend her family's stability to pursue a more authentic romantic connection. This development is about reclaiming her past desires and correcting the course set by a youthful misunderstanding. While she does not possess any special abilities in the context of the narrative, her notable skills include being an excellent cook, a talent that reflects her nurturing side, and she is professionally capable as a working woman in a cosmetics company.