Yuki Katase is the adopted younger sister of Kenji Katase, joining the Katase family at a young age and forming a close sibling bond with Kenji despite their lack of blood relation. Her childhood was marked by significant bonds with Kenji and their friend Hiyori Hayasaka. A pivotal childhood event occurred when Yuki and Hiyori attempted to get a yo-yo from a vending machine for Kenji after he expressed disappointment at receiving a ring. Yuki intended to later give that ring to Hiyori, but Hiyori moved away due to illness before Yuki could fulfill this promise, leaving Yuki with lingering guilt.

Yuki secretly harbors romantic feelings for Kenji, creating significant internal conflict that complicates their sibling relationship. This crush is central to her character. When Hiyori unexpectedly reappears in their lives as a ghost suffering from amnesia, Yuki experiences emotional turmoil. She feels threatened by the renewed closeness between Kenji and Hiyori, forcing her to confront her suppressed feelings for Kenji and the unresolved guilt from the broken childhood promise. The resurgence of these past events profoundly impacts her emotional state and behavior.

Her character development centers on navigating this complex emotional landscape of familial bonds, unrequited feelings, guilt, and jealousy triggered by Hiyori's return and the mystery surrounding Hiyori's condition.