Chigusa Sakai serves as wife to Kantaro Sakai and mother to Yuji Sakai and Miyu Sakai. Raised in an orphanage during childhood, she experienced a pivotal teenage pregnancy with Yuji and his twin brother. Complications during that pregnancy claimed the twin's life and prompted her marriage to Kantaro while both remained students. She currently maintains the household as a housewife while her husband works abroad.

Her physical presence features brown hair typically secured in a ponytail by a mint ribbon and striking olive eyes, noted for their beauty to the point of inciting jealousy among Yuji's friends. Chigusa presents a gentle demeanor yet reveals surprising forcefulness when opposing matters she disagrees with. She possesses a kindhearted nature, high intelligence, and broad-mindedness, enabling her to accept situations without undue concern for details. This trait allows her to welcome Shana's frequent visits unreservedly, despite remaining unaware of Shana's identity as a Flame Haze or Yuji's status as a Mystes containing the Midnight Lost Child.

Her relationship with Shana deepens significantly, evolving into a mother-daughter dynamic where she observes Shana's personal growth. Chigusa becomes a trusted advisor for Shana, offering guidance on matters beyond Alastor's (Shana's contracted Crimson Lord) scope. Alastor himself acknowledges and respects Chigusa's wisdom following a phone conversation. This mentorship manifests practically when Shana, motivated by jealousy over Kazumi Yoshida's bento lunches for Yuji, repeatedly seeks cooking instruction. Chigusa patiently coaches Shana through each step despite Shana's perilous lack of culinary skill. She extends this teaching role briefly to Wilhelmina Carmel, Shana's guardian, resulting in significant kitchen disarray.

Chigusa's guidance also addresses emotional matters. She explains the significance of kisses to Shana and indirectly influences Alastor to permit Shana's adaptation to human society and acceptance of her developing romantic feelings for Yuji. Following Yuji's disappearance during events involving the Snake of the Festival and the creation of Xanadu, Chigusa continues supporting Shana, who becomes a child substitute for her. Her family life involves managing the household during her husband's international work and maintaining a happily married relationship with Kantaro, which he credits to her unwavering determination.