Kayako is a character and the main antgonist of the Grudge movie series. She also appears as the main antagonist of Ju-On: The Grudge, a game which PewDiePie has played.
Kayako was killed by her husband out of jealousy after he discovered that she had feelings for another man. Her death turned her into a vengeful spirit that unleashes its wrath upon anyone who comes upon it. Anyone who encounters her spirit will be consumed by its fury, and death is a certainty. She also has a son, named Toshio Saeki. Toshio is also a ghost just like his mother. He was murdered because he witnessed his father killing his mother.
In Ju-On: The Grudge, the object of the game is to wander through each level, collecting items, whilst avoiding Kayako. She will appear occasionally, prompting a QTE from the player to escape from her.