Sadako is from Koji Suzuki's Ring series of books.

Before the Game

Sadako Yamamura is the antagonist in Koji Suzuki's novel Ring. She returns as the antagonist in Rasen.

In the original novel, Sadako was portrayed with testicular feminization syndrome, meaning that she was anatomically male. In both the novels and movies, it is possible to infer that she is the daughter of some oceanic based entity, making her a quasi-oceanic demigod. Her name combines the Japanese words for "chaste" (sada) and "child" (ko).

Sadako's initial origin story is related in the novel and the first film. She lives on Oshima island with her mother Shizuko, a deeply disturbed woman who also has psychic powers. Shizuko gives birth to Sadako in a cave, where she hopes her child would be carried away by the waves. When she returns the next day, however, Sadako is still there, and so Shizuko keeps her.

As a child, Sadako is painfully shy and rarely socializes with other children. One day, a cousin, Takashi, decides to profit from Shizuko's powers and contacts Dr. Heihachiro Ikuma, a scientist fascinated by psychic powers. Heihachiro and Shizuko eventually form a romantic relationship, and they marry. This union, however, becomes strained when, during a public demonstration of her powers, Shizuko is mocked and insulted by reporters who accuse her of being a fake. Sadako, in defense of her mother, launches a psychic attack which kills the reporter who started the chaos.

By mid-novel, protagonist Kazuyuki had uncovered that those who watch the video die seven days later not of supernatural causes but of a kind of virus. Like any virus, this one too must multiply to survive; hence the imperative to copy the videotape and show it to someone else, that the virus may spread from victim to victim. Kazuyuki believed this to be merely a convenient instrument of Sadako's revenge. In fact it was much more.  Sadako was thrown ino the well by a young doctor named Nagao Jotaro. Nagao was also the last-known patient in Japan to be treated for smallpox.

Out alone with Sadako one day, Nagao suddenly finds himself in a lustful rage (which he was to later blame on a smallpox-induced fever) and sexually assaults her. It isn't until afterwards that a shocked Nagao learns Sadako's secret. She had been born with a condition known as Testicular Feminization Syndrome, meaning that while she outwardly appeared female, Sadako's chromosomal makeup was actually that of a man. She was also born without a uterus, and possessed (as Nagao saw) a pair of testes.

Suddenly ashamed at having been discovered, Sadako mentally screams I'LL KILL YOU! and launches a telepathic attack. Nagao retaliates by strangling her and throwing her into the well...but of course, we know that she was not yet dead. Within the well, Sadako forces her considerable will upwards, where it hangs like a menacing presence until, several years later, a series of rental cabins are built over that very spot. Her mental projections find the perfect medium in videotape, which burn themselves onto the film and await the time that they can be released by someone viewing the tape.

All of this explains the how behind the "cursed videotape," but it still doesn't address the why. Was Sadako's aim in creating the tape merely to exact revenge for her death? No.

Sadako possessed absolute telekinetic power on a cellular level. She was able to use the medium of the smallpox virus, transmitted to her by her violation at the hands of Nagao, to create something altogether different. Ando Mitsuo, the protagonist of Rasen, discovered the virus to in fact be seven parts smallpox, three parts human genetic code. Via her paranormal abilities, Sadako was able to combine her own genetic material with that of the smallpox virus, creating an invasive mechanism capable of transmitting her DNA to the host/victim. This DNA would then replicate within the host, leading to (if the necessary conditions were met) the birth of a duplicate Sadako -- or, more strictly speaking, Sadako version 2.0. In Rasen, we see that the reborn Sadako has both a womb and testes, which causes her to refer to herself as "perfectly dual-gendered."

Powers

Sadako's most common method of killing in the books is the video curse. Within the book, it is discovered to in fact be a virus. When someone watches the cursed tape (or something else carrying the curse) some of their DNA is changed to become that of the Ring Virus (i.e. a hybrid of Sadako's DNA and that of the smallpox virus). This travels through their body and in most cases causes a sarcoma to form on one of the arteries of their heart. If the curse has not been appeased within seven days, the sarcoma detaches from the artery and clogs it, causing heart failure.

Sadako exhibits a variety of abilities (mentioned above), including telekinesis, the ability to kill people instantly with psychic powers, healing abilities, ESP, and possibly the ability to split herself into two beings and merge them again.  Due to her curious new biology, Sadako also seems to be able to resurrect people.

She had remained alive until she imparted her curse onto the tape, meaning that she also had superhuman endurance and longevity, as well as inedia (the ability to live for extended periods without nourishment).

Lastly, she imparts her curse onto the video tapes, implying the ability of projected thermography.

Current Updates

Sadako arrived in the vortex confused, but somewhat pleased that she was in a world where she was no longer the only one there.  At first, she tried to start her acting career again, as she had in her world, to limited success.  So when legendary metal band Dethklok needed a healer for their older lead guitarist, she answered the call.  She's stayed on, and has grown to truly love the band as her own sort of odd adopted family.  359, the woman who hired her, is now one of her nearest and dearest friends. She's also particularly fond of Teja Wartooth, and considers him to be her son despite their lack of genetic relation. 

Sadako was dating and living with Sweeney Todd, in love with him despite the fact that she knew he'd always remain emotionally distant.  When the vortex took him away, she went on a minor rampage (for her), going to the west of England and killing a small town.  However, she was surprised to find a secret admirer in the form of Alexia Winters, someone she'd long considered a friend.  She fell in love with him and was His due to a sigil in the form of a scar on her wrist.  She was surprised to find that her own son, Teja, killed Alexia to get out of owing the demon his soul.  She mourned for many months.  Eventually, when detective L arrived, she struck up a tentative friendship with him.  He inspired her to chase her dream of being an actress, and she landed a role as a hostess on a popular morning television show in Tokyo.  She also chased him, and they started a very chaste romantic relationship. She was even more thrilled when the child version of her son Teja Wartooth showed up.  She called him Tej.   Upon the return of her first love, Sweeney Todd, Sadako returned to him with the intention of leaving L.  When she went to try, she found that both he and the younger version of her son had disappeared.  Upon Sweeney's disappearance again, Sadako swore off relationships and functioned as a benevolent matriarch for the Dethklok brood, healing them when they need it and offering advice when they don't.  Then, suddenly, Alexia returned, brought back by Teja.  Alexia went on a rampage, killing everyone that mattered to Teja, including Sadako.  He said that he loved her as he killed her, and she believed him.  When she returned to life, she ran to him, resuming their relationship where they'd left off.  She remained with him until her friend Clavis, a psychic like her, pointed out that she loved her son Teja more than Asmodeus.  Sadako had tried to be Teja's mother, but since she hadn't raised him, the love had grown more romantic than anything else.  She realized that was true, and since Teja had recently gotten divorced, confessed her love.  She lived with him for a time before he returned to the pits.  Now she lives alone in Japan, focusing more on her healing abilities, getting along with her father and siblings (Ikuko Kada and Jae-Sun Kim), and acting.  She's recently started dating Mozzie, and while they're an unlikely couple, they're very happy together.

Useful Links

Sadako's background is confusing as hell, I know - here's a really awesome timeline of her activity in the books.

Player Information

Sadako is played by Sarah. Sadako's PB is Chiaki Kuriyama. Sadako's journal is wheniamqueen. Most information taken from Sadako' wikipedia entry, as well as theringworld.