Evelyn Borden was the biological mother of Samara Morgan and for a majority of her life a patient in a mental institution. She is mentioned and shown in The Ring 2 which explains it all: she came to a Catholic hospital pregnant with Samara, claiming that the father was a demon from the sea. One night, she attempted to drown Samara, but the nuns took the baby away from her arms. She is believed to be insane because she claims her child is not human at all and is fully evil. She was locked away in a mental institution and died before she could see the day Samara comes at rest.
Evelyn later died from an unknown cause. Her ghost began to haunt a nearby church. More about her backstory was revealed in Rings. Evelyn wasn't raped by a sea demon who fathered Samara, but a priest named Galen Burke at the church. Burke kept Evelyn locked away beneath a bell tower until she was found and taken to the hospital where she gave birth to Samara. Burke was eventually killed by Samara. It is implied that the trauma of her pregnancy is what caused Evelyn to think her child was possessed by a sea entity. How Samara obtained her powers is still unknown.