One of the first things that stands out about Sadako Yamamura from the original novel and film adaptations is how she became the blueprint for a very specific kind of ghost in the late 90s and early 2000s. She's not a vengeful spirit tied to a place, like a traditional onryō haunting a well or house. Her curse is technological, transmitted through a videotape, which at the time felt incredibly modern and plausible. That's a huge shift from, say, the ghosts in 'Kwaidan' or the pervasive sadness of a yūrei. Her motive is also less about personal vengeance against a specific wrongdoer and more about a blind, almost biological need to spread her pain. It's a contagious curse.
Comparing her to something like the Pontianak or the Penanggalan from Southeast Asian folklore highlights how Sadako is a creature of narrative, not just legend. Those spirits have set rules and appearances rooted in cultural beliefs about women who died in childbirth. Sadako's rules are created by the story—the seven days, the copying of the tape—which makes her feel like a puzzle to be solved, a horror movie villain. She's less a folkloric warning and more a narrative engine, which is probably why she crossed over internationally so effectively. Her look, the hair and the white dress, is now iconic, but it's interesting that it's a cinematic invention that then fed back into the aesthetic of horror.