After losing her mother at a young age, Yuka's household welcomed Hibiki as her stepmother, who cared for her like her own daughter and made continuous efforts to bridge the emotional gap between them. With almost no memories of her real mother, Yuka struggled to recognize anyone as a maternal figure. In truth, Hibiki was a sexually liberated bisexual woman who had harbored intense feelings for Yuka even before marrying into the family. Her unusual expressions of affection took a dramatic turn one day.