Sadako's evil spirit is based on the Japanese concept of onryō, or "vengeful ghosts." Onryō were thought to be the souls of those who died with extreme hatred, particularly women. They all have a specified appearance: pale women with long, dishevelled black hair wearing white burial clothes. Sadako herself is based on two famous onryō: Okiku of Banchō Sarayashiki, who was murdered and thrown down a well by the samurai she spurned, and Oiwa of the Yotsuya Kaidan, who was fooled into drinking poison and murdered by her husband.

Sadako is very powerful, inheriting her psychic powers from her mother and possibly her unknown father in the films. Her most notable power is that of Nensha, an ability which allows her to telekinetically burn images onto surfaces or into a person's mind. This allows her to create the videotape while trapped in the well. The cursed tape allows her to appear as a ghost and extend her Nensha powers to others." ~ The Ring Wiki