Kayako Saeki is the main antagonist of the Ju-On and The Grudge horror franchises. Kayako's fictional history alternates slightly between continuities, but all depict her as the very attractive yet unfortunately vengeful ghost of a woman killed by her husband, Takeo, along with their son Toshio, in a murder–suicide that happened after he came to believe she was having an affair. After the crime, the spirits of all three are bound to their family home, haunting and killing all who enter in the following years.

In Japanese traditional beliefs and literature, onryō are a type of ghost believed to be capable of causing harm in the world of the living, injuring or killing enemies, or even causing natural disasters to exact vengeance to "redress" the wrongs it received while alive, then taking their spirits from their dying bodies. Onryō are often depicted as wronged women, who are traumatized, envied, disappointed, bitter, or just furious by what happened during life and exact revenge in death. These kinds of ghosts appear extremely vengeful, ruthless, heartless, brutal, cruel, deranged, egotistical, selfish, bloodthirsty, and cold-hearted.