Rachel Dawes was a lawyer and assistant to the District Attorney in Gotham City.
Batman Begins
As a child, Rachel was good friends with Bruce Wayne and played with him at Wayne Manor. She was present when he fell down a well and handed him an old arrowhead she had found in the garden. Years later, after Bruce returned from Princeton University, she saw him again and persuaded him not to go to the hearing of Joe Chill, the murderer of Bruce's parents. He went anyway, and took a gun with the intent of killing Chill, which Rachel grew angry at him for. She showed Bruce the poverty of Gotham and told him the city didn't stand a chance if the good people did nothing.
Several years later Rachel was assistant to the DA Carl Finch, when Bruce returned to Gotham after being presumed dead. She attempted to prosecute Victor Zsasz, a man working for Carmine Falcone, but the corrupt psychiatrist Jonathan Crane got him off on an insanity plea. When she confronted Crane about it, Finch told her to back off. As she was travelling on the subway, Falcone's men attempted to mug her, but Batman saved her and gave her pictures to use as leverage against the corrupt judge Faden. The next morning, Falcone was found tied up and implicated in drug smuggling, and he was arrested.
Rachel realised they had Falcone banged to rights, but Crane submitted another insanity plea. Around that time Finch went missing, so after delivering Bruce's birthday gift she went to Arkham Asylum to see what had happened. She found Falcone babbling about a scarecrow, and confronted Crane again. He took her to a basement and sprayed her with fear toxin, before strapping her to a table. However Batman appeared and saved her again, taking her to the Bat Cave in The Tumbler and injecting her with antidote. Alfred Pennyworth then drove her home, and she woke up with instructions to get the antidote to James Gordon for mass production.
She went to The Narrows and found people going insane with fear. She injected a young boy, and noticed Ra's al Ghul and his men with the microwave emitter. She then escaped from the Scarecrow by hitting him with a taser. Batman then revealed his identity as Bruce Wayne by repeating a quote back to her. She visited the remains of Wayne Manor and told Bruce she couldn't be with him, as he was more comfortable as Batman than his normal identity.
The Dark Knight
Several months later Harvey Dent was elected as the new DA, and Rachel began a relationship with him. Together they tried to take down Sal Maroni, eventually succeeding when they used Mr Lau in a reco case. However the Joker started making threats on public figures, and invaded a fundraiser where he took Rachel hostage. He threw her out of a window, but Batman saved her.
Later, after Harvey had revealed himself as Batman, Rachel was kidnapped and put in a warehouse full of explosives. Batman was given the wrong location to her whereabouts, so she was killed in the explosion. Before she died, she wrote a letter to Bruce explaining she was going to marry Harvey.