Rachel is Bert's "special friend." The two are, indeed, friends, but there are always hints that they are crushing on one another. She's a schoolteacher, too, but is convinced at the beginning of Inherit the Wind that Bert was wrong in breaking the law to teach Darwin.
She's not so dead set against Darwinism as some other people are, though. See, Rachel is highly influenced by the Reverend Jeremiah Brown, her very powerful father. And he's outspoken about the fact that she will go to hell if she defends Bert. Sweet, huh?
You might say that Rachel is pretty easily influenced. While Bert is frightened of facing jail time for his actions, he maintains his convictions throughout the play. Rachel, on the other hand, is easily bullied by the more prominent people in town.
However, she does manage to resist, in a way. Rather than speaking up for Bert, she falls silent on the witness stand, refusing to testify against him.
And, despite her weak will throughout most of the play, Rachel is probably the most dynamic character in it. By the end of Act III, she has found the strength to escape the clutches of her crazy dad:
Bert, it's my fault the jury found you guilty. (He starts to protest) Partly my fault. I helped. (RACHEL hands BERT a book) This is your book, Bert. (Silently, he takes it) I've read it. All the way through. I don't understand it. What I do understand, I don't like. I don't want to think that men come from apes and monkeys. But I think that's beside the point. [. . .] Maybe what Mr. Darwin wrote is bad. I don't know. Bad or good, it doesn't make any difference. The ideas have to come out—just like children. Some of 'em healthy as a bean plant, some sickly. I think the sickly ideas die mostly, don't you, Bert? (BERT nods yes, but he's too lost in new admiration for her to do anything but stare. He does not move to her side. DRUMMOND smiles, as if to say: "That's quite a girl!" [. . .]) (III, 563-88)
So, Rachel is designed as a stand-in for all the people who are swayed, even if they don't want to be, by others' influences. But, in the end, she embraces the light side of life: she reconciles new ways of thinking with her old beliefs. You go, girl.