Kaya is the Miyanoshita's family pet, a Bombay black cat. It is stated that Kaya was a stray cat that came to them on the day of Kayako Miyanoshita's funeral.
The Miyanoshita family is very protective of him and consider him a family member, especially Keiichirou because of his affinity with his mother. He was possessed by Amanojaku in Episode 1 and returned to normal in Episode 20.
Kaya is shown to naturally have heterochromia, a condition in which the eyes are two different colors, his right eye is blue, while the left is yellow. Kaya appears to have some supernatural affinity, as he is naturally drawn to the Old School House in Episode 1 and is drawn to the hiding place of the Ghost Diary.