She was sneaky as hell when she first came to Collinwood, against all reason and sense.
I figure that when Rachel first got wind of the fact that there was somebody in the Tower, she feared it was a child locked up. She had seen enough of that sort of thing at her alma mater to make it a possibility that would immediately occur to her. And the doll that Beth took to the tower room would have encouraged this idea.
In Tim's defense, [spoiler]he was under Dirk's power, I think (seems to me we didn't see the actual bite),[/spoiler] and lurking around his mind there may have been an order against giving Rachel any truly effectual assistance. A weak argument, and not one that I really believe.
Poor Rachel. Magda asks her if she's brave, and she says, "Not very." But she was. She had the worst imaginable luck - nothing
ever went right for her in her whole life - starting with conception, when she got shortchanged in the brains department and was instead given the sort of beauty that would draw Trask's attention to her - but she kept on trying to do the right thing.