Chiyako Koishikawa is Yuu Matsuura's mother and becomes the second wife of Jin Koishikawa, Miki's father, following the unusual family swap that serves as the premise of Marmalade Boy. Her maiden name is Namiki.

Chiyako is portrayed as a gentle and soft-spoken woman, possessing a calm and warm demeanor that contrasts with the more outspoken personality of Rumi, Miki's biological mother. She is described as having a "cool big sis" attitude toward Miki, deliberately choosing not to be pushy or overbearing, instead allowing a natural and caring relationship to develop with her new stepdaughter. This gentleness and patience make her an approachable and comforting presence within the newly blended and chaotic household.

Professionally, Chiyako works in new product development at a cosmetic company, indicating she is a capable and modern career woman. Her career is shown in the story when she proposes that Yuu appear in a promotional commercial for her company, demonstrating her involvement in her work and her desire to connect her professional and family life.

Her motivations are rooted in the pursuit of personal happiness and the well-being of her family. The decision to divorce Youji Matsuura and marry Jin Koishikawa, a man she fell in love with during a trip to Hawaii, is presented not as a frivolous act but as a sincere, albeit unconventional, choice for love. As a mother, she is supportive and loving toward Yuu, and she works to build a positive relationship with Miki, showing a strong desire to make their unusual living situation harmonious.

Her key relationships define her role in the story. As Yuu's mother, she is a source of love and stability, though her past holds a significant secret. A central piece of her backstory involves a tragic event: she was previously pregnant with Jin Koishikawa's child before her marriage to Youji, but the baby was stillborn. A letter from her mother regarding this painful part of her history becomes a source of major confusion for Yuu, who mistakenly believes it refers to him. Her marriage to Jin is the foundation of the entire plot, creating the circumstances that force Miki and Yuu to live together. In this new family structure, she becomes Miki's stepmother and is consistently kind and gentle toward her, helping to ease Miki's initial shock and resentment about the situation.

While Chiyako is a relatively static character who does not undergo a dramatic personal transformation, her presence is a catalyst for the growth of others, particularly Yuu. The discovery of her past and the misunderstanding it creates forces Yuu to confront his own identity and his feelings about his family. She represents the adult perspective on the story's central theme: that love can be complicated and that happiness sometimes requires defying social conventions. Her notable abilities lie not in any physical or special skill, but in her emotional intelligence and patience. She navigates the complex emotions of her new family with a quiet grace, and her gentle nature serves as a calming influence that helps hold the two families together, even when the younger generation faces turmoil.