Kaoru Aono attends the same high school as Shō Kurogane as a human student and is his childhood friend, sharing a close bond demonstrated by offering him shelter under her umbrella in the rain and voicing concern for his safety. She collapses from shock during a harpy attack. Later, she befriends Yama Rikudō, the transfer student secretly a demon hunter, gifting Yama an umbrella as a gesture of kindness.

Supernatural forces entangle Kaoru when the demonic Headmistress manipulates her and classmate Sawaguchi into a storage shed under false pretenses of relocating textbooks. Inside, a parasitic nautilus latches onto her neck, subjecting her to mind control. During Yama’s clash with the possessed group, Kaoru briefly shatters the parasite’s hold at a pivotal instant. She leaps to shield Shō from an attack, inadvertently catalyzing events that annihilate the mind-controlling parasites. Post-conflict, she embraces Yama’s true demonic nature without reservation.