Yuka Midarezaki, born Reiko Himemiya, originates from the Himemiya lineage infamously dubbed the "Demon's Family." She endured the ritualized abuse known as "Solitary Doll," a tradition targeting the weakest member, usually the youngest, for stress relief. Her biological sister, Senko Himemiya (later Chika Midarezaki), participated alongside other family members.

Recruited into Operation Cozy Family, she assumed the role of eldest daughter in the Midarezaki household. Despite her traumatic past, Yuka retains a gentle and calm demeanor. She forms a profound attachment to her adoptive father, Ōka Midarezaki, constantly seeking his approval about her appearance and harboring deep fears of his abandonment. This drives actions like joining family schemes to protect their dynamic, exemplified by an incident where she dehydrated an alien suitor by consuming all the household water.

Attending elementary school, Yuka befriends a classmate named Yamamoto. Their interactions involve him offering pudding bribes for academic assistance, suggesting her fondness for the dessert. She consistently struggles to correctly recall another classmate's surname, Yamaguchi, mispronouncing it each time.

A significant bond exists with Teika Midarezaki, the family's talking lion. She frequently rides him, seeks comfort in his presence, and uses him for solace. Teika reciprocates this affection, addressing her respectfully and permitting her to cuddle or ride him. Her relationship with Chika remains complex, rooted in their shared abusive history, though Chika's integration into the Midarezaki family and Gekka's implied protection indicate a shift from past victimization.

Her development culminates in inviting Yamaguchi to her home, signaling growing confidence and social integration beyond her immediate family.