Rachel Black is a member of the Quileute tribe, the older sister of Jacob Black, the twin sister of Rebecca Black and the daughter of Sarah and Billy Black. After her mother died in a car crash, Rachel received a scholarship to Washington State and moved away from her father and brother. In Breaking Dawn, however, she has finished her degree and moved back to La Push, where she was imprinted on by Paul. She is a good influence on him and likely to be very calm and patient, to counter his aggressive and volatile personality.
In Twilight, at the beach, Jacob tells Bella that Rachel got a scholarship to Washington State college. Jacob did not tell Bella that his older twin sisters did not feel comfortable about living in La Push after the death of their mother, Sarah Black.
After graduating early from Washington State college, Rachel moved back to La Push to live with her father and younger brother. She was unaware of the existence of shape-shifters and Billy and Jacob were fully prepared to conceal the secret from her, but then Rachel met Paul at First Beach and he imprinted on her, making her eligible to know about the shape-shifters' existence. Though Paul is four years younger than her, Jacob describes their relationship as "love at first sight".
Jacob and Billy are both frustrated by Paul's presence in their house, but are glad to have Rachel home as she and Rebecca both feel uncomfortable in La Push. Jacob suggests that she go and stay at Paul's house, but Billy refuses, as he missed her when she was away and wants his daughter to live with him for as long as possible. He does "escape" to the Clearwaters' house most days though, to avoid Paul and to keep the recently widowed Sue Clearwater company.
At the beginning of the second book of Breaking Dawn, Jacob, who is already angry and irritable about Bella's prolonged absence, is annoyed at Paul for hanging around his house even when Rachel is out, but says that Rachel would "kill" him if he hurt her boyfriend. When he discovers that Paul is eating his Doritos, Jacob loses his temper and punches him in the face, breaking his nose (which heals almost instantly).
Rachel obviously cares a lot for her younger brother, as she would rather continue sleeping on an air mattress than see Jacob leave. It is assumed that she cares a lot for Rebecca too, as they are twin sisters. Billy loves Rachel dearly as he accepted Paul into his house, Paul being the only reason Rachel stayed in La Push for so long.