Yuiko Amaha is the mother of Miu Amaha, a student at Yuihime Girls' Academy. Her youthful appearance frequently causes others to confuse her for Miu's older sister. Yuiko lives with Miu in a house near the school, sharing their home with two cats named Domon and Karin. Business commitments require Yuiko's husband, Miu's father, to reside in a distant town, physically separating him from the family household.

Yuiko herself is an alumna of Yuihime Girls' Academy. During her time as a student, she formed a friendship with Ranka Sena, who is now the principal of Yuihime Girls' Academy and the mother of Airi Sena.