Kaoru Aono is a high school student and the childhood friend of the protagonist Shō Kurogane. She has brown hair and is established as a kind and considerate person early on, as seen when she invites Shō to share her umbrella while walking together in the rain. Her role in the story is primarily as a supportive figure and a classmate to both Shō and the supernatural protagonist Yama Rikudō, whom she befriends by offering her an umbrella.

Kaoru's importance to the narrative is most clearly demonstrated during the climax at the school's storage shed. After she and her friend Sawaguchi are lured there and subsequently placed under a demonic mind-controlling spell, a small monster attaches itself to her neck to control her actions. While Yama and Shō are forced into a deadly standoff by the villain, Kaoru manages to exercise enough willpower to break free from the demon's control. She crushes the creature holding her hostage and bravely puts herself in harm's way, rushing in front of Shō's sword to prevent him from harming Yama or himself. This decisive action breaks the spell over the others and allows Yama to confront the Great Satan.

Throughout the OVA, Kaoru develops from a passive classmate caught in supernatural events to an active participant who demonstrates significant inner strength and courage. Her loyalty to her friends ultimately outweighs the demonic influence placed upon her. By the end of the story, despite learning that Yama is a supernatural being from the underworld, Kaoru, along with Shō and their other friends, fully accepts her and welcomes her into their group. She does not possess any special combat abilities or supernatural powers, but her value comes from her empathy, her willpower, and the normal human perspective she brings to the group.