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Sadako Yamamura is a nice, quiet, shy Japanese girl who just wanted to become an actress.

...well, that's half right, anyway.

Half psychically-endowed human, half superpowered, sociopathic monster, Sadako may have been fathered by some sort of supernatural sea-creature, or possibly by "the sea" itself. Her mad mother never told her either way, and her adoptive father and physician always claimed that she was his--even as he was plotting to kill her.

Here is her history, according to the Wikipedia for Ringu 0, the continuity I am drawing her from.

Sadako Yamamura's mother was a woman named Shizuko, living on Oshima island. She was despised by the locals for her psychic powers and the fact that she'd sit for hours by the shoreline muttering incomprehensibly at the sea. Shizuko birthed Sadako in a cave, where she hoped her child would be carried away by the waves. When she returned the next day, Sadako was still there and was kept.

As a child, Sadako was painfully shy and rarely socialised with other children. One day, a cousin, Takashi, decided to profit from Shizuko's powers and contacted Dr. Heihachiro Ikuma, a scientist fascinated by psychic powers. Heihachiro and Shizuko's relationship soon deepened and they married. This union, however, became strained when during a public demonstration of her powers, Shizuko was mocked and insulted by reporters who accused her of being a fake. Sadako, in defence of her mother, launched a psychic attack which killed the reporter who started the chaos. The Yamamuras' lives were further complicated by Sadako suddenly splitting into two people; one kind hearted and timid, the other violent and psychotic. The good Sadako was allowed to live on whilst the other was kept in seclusion and forced to take growth inhibiting drugs. These events proved too much for Shizuko to bear, so she threw herself into mount Mihara.

At the age of 19, the good Sadako joined an acting troupe in Tokyo where she was immediately looked upon with suspicion as a number of actors began dying under mysterious circumstances (which was actually the fault of the evil Sadako using her twin as conduit). Sadako entered a relationship with a sound editor named Toyama, the only person who showed her any acceptance. A set of calamitous circumstances result in Sadako being beaten to death by the acting troupe and taken to Dr. Ikuma's home where they intend to finish off the evil Sadako. The plan fails disasterously, resulting in the two Sadakos merging into a single entity of pure destruction. Sadako kills the entire troupe, including Toyama. Dr. Ikuma beats her over the head with an axe and throws her down a well, where she survives by sheer willpower for over 33 years.

In the above timeline, she has been trapped in the well for eight months and still (relatively) has her sanity, though it is crumbling. Her two personalities have not merged yet, and Sadako's monstrous sister/half (hereafter referred to as S.) tends to come out (or simply use Sadako as a conduit) whenever Sadako is under stress. When that happens, things tend to get horrific and bloody very quickly. Sadako lives in terror of S., and will thus do her best to avoid any situation where she might come into conflict with someone or be angered, frightened or overly irritated around other people. She's not a ghost--yet--or the embodiment of a deadly self-cloning virus. Rather, she is a schizoid, tormented being who wants acceptance as a normal human, but must constantly deal with the influence of her inhuman half.

http://en.wikipedia.org/wiki/Ring_0:_Birthday
http://en.wikipedia.org/wiki/Sadako_Yamamura
http://www.theringworld.com/ring0.php

PB: Nakama Yukie