Kayako is Satsuki's and Keiichirou's deceased mother. Through her diary it is revealed that she was an avid exorcist during her childhood. She possesses Momoko two times throughout the series to aid Satsuki against the Piano Ghost and Kutabe and once helped Satsuki and the others to escape from a powerful haunting called Anamnaneki. Her maiden name was Kamiyama.

(Source: Wikipedia)