Natsuki Aoi is a female teacher at Rokukeikan Academy with hip-length black hair and brown eyes, standing 171 cm tall with body measurements of 101-65-90 cm. She is also a keyhole affiliated with an organization, serving as the sole adult within her group who strives to maintain calm and cohesion under high-stress conditions. Within this organization, she operates as a Woman In Black, demonstrating unwavering loyalty by following orders without question, even to a potentially self-destructive degree. Her capabilities as an Action Girl include combat and strategic planning skills that exceed her initial appearance. In her story route, she enters a Teacher-Student Romance with Takato Keisuke. She acts as the Token Good Teammate in her organization, expressing guilt and regret over her role in a twisted plan that harmed many civilian students, and she attempts to assist Takato to the best of her ability in the true ending. One scenario designed to humiliate her involves a Manchild situation where she is forced to wear a poorly-fitting bib and diaper, then given a milk bottle before sexual intercourse, emphasizing her infantilization as an older teacher. In the anime adaptation, she achieves a Happily Ever After by escaping the facility with others after the main villain is killed. She remains on a beach with Takato Keisuke and Rika Makiba, engaged in continuous sexual activity with both.