Sayo Shiga is a housewife with brown hair and brown eyes, possessing physical measurements of 88-64-90. She entered the Shiga family when Chikara Shiga married her after his first wife's death, becoming stepmother to his son, Kaito. Sayo harbors a strong aspiration to have a child, a desire her husband consistently rejects.

Prioritizing her roles as a dutiful wife and stepmother, Sayo suppresses her own needs to preserve family harmony. She endures demeaning treatment from Chikara, who treats her more like a sex worker than a partner during intimacy. Aware that he no longer loves her, Sayo nevertheless lacks the self-confidence or assertiveness to alter her situation. This passivity extends beyond the home, as she allows herself to be dominated by figures like Tamae, the parent-teacher association chairperson.

A pivotal moment occurs when Sayo discovers Chikara's infidelity with a younger woman. Subsequently, she embarks on a secret relationship with her stepson, Kaito. Though initially wracked by guilt, this relationship sparks a visible positive transformation in her demeanor and outlook.

Her background centers on joining the Shiga household through marriage, where she strives to create a stable family environment despite facing underlying tensions and emotional neglect from her spouse.