Yūko Mizuhara is Ichika Mizuhara's biological mother. Following her remarriage to Masaya Mizuhara, she becomes Tomoya Mizuhara's stepmother. This marriage forms the blended family central to the series, establishing the step-sibling relationship between Ichika and Tomoya.

In the OVA special "Oneechan ga Kita: Hajimete no… Kitaa!", depicting Ichika and Tomoya's initial meeting, Yūko plays a pivotal role. She brings Ichika to meet Tomoya and Masaya after the death of her first husband, Ichika's biological father. During this emotionally charged introduction, while Ichika processes her grief, Yūko gently encourages the hesitant girl to interact with her new stepbrother, showcasing her support in facilitating the new family dynamic.

Yūko Mizuhara has black hair and red eyes. She fulfills the parental role alongside Masaya within the Mizuhara household throughout the series. Her character maintains this consistent presence without significant narrative expansion beyond establishing the family structure.