Chigusa Sakai is the mother of the main protagonist, Yuji Sakai, and the wife of Kantaro Sakai, who works outside the country. Her background includes being raised in an orphanage during her childhood. As a teenager, she experienced a complicated pregnancy with twins, one of whom was Yuji. The other twin did not survive the birth, an event that led to her marriage to Kantaro while both were still students. She maintains the household as a housewife in her husband's absence.

In appearance, Chigusa has brown hair that she typically secures in a ponytail with a mint-colored ribbon and olive eyes. Her beauty is noted to be considerable, even inspiring jealousy among Yuji's friends. Her personality is a blend of gentleness and surprising forcefulness; she is kindhearted and possesses high intelligence and broad-mindedness. This combination allows her to strongly object to things she disagrees with while generally accepting situations without undue concern for details. This trait is exemplified by her complete lack of issues with Shana, a girl who is a stranger to her, frequently visiting her home under the pretense of training with Yuji. She remains entirely ignorant of the supernatural true nature of both her son, who is a Mystes containing the Midnight Lost Child, and Shana, who is a Flame Haze.

Chigusa's primary role in the story is as a source of wisdom and guidance, particularly for Shana. She becomes a trusted maternal figure and advisor for Shana on matters that Alastor, the Crimson Lord contracted to Shana, cannot counsel her on. In a notable instance, Alastor himself acknowledged and bowed to her wisdom following a telephone conversation in which he posed as Shana's guardian. Her guidance extends to emotional and social matters, including explaining the significance of a kiss to Shana and indirectly influencing the more traditional Alastor to permit Shana's adaptation to human society and her developing romantic feelings for Yuji.

Her key relationships are central to her role. Her bond with Shana deepens significantly, evolving into a mother-daughter dynamic as she observes Shana's personal growth. Practically, this manifests in Shana repeatedly seeking Chigusa out for cooking lessons. Motivated initially by jealousy over a classmate, Kazumi Yoshida, making bento lunches for Yuji, Shana asks Chigusa to teach her. Despite Shana's perilous lack of culinary skill, Chigusa patiently coaches her through the steps. She also briefly attempts to teach Wilhelmina Carmel, Shana's guardian, a task that results in a significant mess in the kitchen. Within her family, she maintains a happily married relationship with her husband, who credits her unwavering determination for their strong partnership. Following Yuji's disappearance during a major supernatural event, Chigusa continues in her role by supporting Shana, who becomes a kind of substitute child for her.

Regarding notable abilities, Chigusa has no supernatural powers. Her strengths are entirely grounded in her personal qualities. She is an excellent cook and a patient, wise teacher. Her most significant ability is her profound, almost preternatural wisdom and emotional intelligence, which allows her to provide sound advice and guidance even to beings far beyond the scope of her normal human understanding. Her capacity for acceptance and her strength of character make her an anchor for the other characters.