Constantly accompanying the young boy Yuu as a ghostly presence, Satsuki is revealed during the story's climax to be his sister. Neither character's name is spoken in dialogue, leaving their identities to be confirmed by production credits.

Satsuki arrives as one of several transfer students in Amelia's class during the second OVA. Like the other newcomers, she receives minimal development compared to the established cast. Her ghostly nature and connection to Yuu remain inherent traits, unexplored through backstory or character development within the animation.

Her presence functions primarily as a visual extension of Yuu, lacking significant individual actions, dialogue, or narrative focus beyond this bond. The OVA provides no insight into her background, the circumstances of her ghostly state, or any development independent of her role as Yuu's sister.