Kaya is a black Bombay cat with heterochromia, displaying one blue eye and one yellow eye. Originally a stray appearing on the day of Kayako Miyanoshita's funeral, the cat was adopted by the Miyanoshita family and especially cherished by Keiichirou Miyanoshita.

During an exorcism in Episode 1, Satsuki Miyanoshita and her friends accidentally sealed the demon Amanojaku within Kaya's body after their intended target, a camphor tree, was destroyed. Trapped within the cat, Amanojaku initially showed hostility towards the children but slowly formed an attachment, particularly to Satsuki and Keiichirou. Despite his antagonistic nature, the demon frequently provided vital aid against supernatural threats.

Even before possession, Kaya exhibited inherent supernatural sensitivity, guiding the children to key locations like the old school building and the hiding place of Kayako's Ghost Diary, hinting at a latent spiritual connection or Kayako's lingering influence. Throughout the series, Keiichirou maintained a strong bond with the possessed cat, consistently calling it "Kaya" and viewing it as a comforting link to his deceased mother, rather than acknowledging Amanojaku.

The possession lasted until Episode 20, when Amanojaku was separated from Kaya's body during the final confrontation with the ghostly threat. Freed from the demon's influence, Kaya reverted to an ordinary cat. Keiichirou expressed sadness at Amanojaku's departure, revealing he had grown fond of the demon's presence within his pet.