Hiyori Hayasaka is Kenji Katase's childhood friend, their bond originating when Kenji found her as a lost, crying girl in a park. Years after her family moved away, she reappears unexpectedly as a spectral figure emerging from Kenji's closet. Her ghost-like form is transparent and intangible, coinciding with significant memory gaps that leave her unable to recall her arrival or recent past.

Her childhood harbored unexpressed affection for Kenji. This included an incident where she and Yuki Katase, Kenji's adopted sister, attempted to get a yo-yo from a vending machine as a gift after Kenji expressed disappointment over receiving a ring. Hiyori moved away before Yuki could give her the ring intended as a token of their friendship, contributing to unresolved feelings and secrets among the trio.

Kenji and Yuki strive to help her recover her lost memories and understand her spectral state. Hiyori exhibits emotional vulnerability and a tendency to use made-up words. Despite her ghostly condition, she engages in everyday activities like visiting amusement parks and riding rollercoasters, highlighting inconsistencies in her physical presence.

Her journey involves uncovering the mystery of her condition and confronting buried emotions. Key developments include the gradual recovery of fragmented memories related to her life after moving away and the resurfacing of her unresolved romantic feelings for Kenji. Her relationship with Yuki is also explored, revealing mutual guilt and complicated bonds stemming from their shared childhood connection to Kenji and the unresolved yo-yo incident.