Rachel is one of the central characters of the novel. You might even think of her as the romantic heroine, since she's the pretty girl that everyone wants to marry. Yet we never hear the story from her point of view. Why is that? Well, for one thing, it would spoil the suspense: she knows from the beginning that Franklin Blake took the Moonstone, although she doesn't know the reason why. And she keeps that knowledge to herself for most of the book, right up until Franklin forces her to speak out.
That reticence, or unwillingness to speak out, is one aspect of Rachel's character that is continually brought up. She's not chatty or gossipy like the other girls of her age. She doesn't tend to ask for advice. She is stubbornly secretive. Is this a good thing, or a bad thing? On the one hand, it makes her very independent. She doesn't rely on anyone else to give her advice. When she has a difficult decision to make, she takes her time, considers her options, and makes up her mind on her own.
Being self-reliant is a good thing, right? Isn't it good that she doesn't tattle-tale on people she likes, even when it means getting in trouble herself? Sure, but up to a point. Think about how much time and trouble would have been saved if she had just confronted Franklin the morning after the diamond was stolen, and said, "Look, Franklin, I SAW you take it!" They would have figured out pretty quickly what must have happened, and the novel would have ended about three hundred pages sooner.
So Rachel's reticence is partly a plot device: for the story to go forward, it's necessary that Rachel should withhold what she knows. But it's also a comment about gender and femininity. When Gabriel Betteredge first describes her, he compares her to "other girls of her age," saying that "she judged for herself, as few women of twice her age judge in general" (188.8.131.52). Betteredge describes this as a "defect," though – it's not a compliment to say that a young woman is "independent" during the Victorian period. Mr. Bruff thinks so, too, generally speaking. But in Rachel, he considers it a virtue:
This absolute self dependence is a great virtue in a man. In a woman it has the serious drawback of morally separating her from the mass of her sex, and so exposing her to misconstruction by the general opinion. I strongly suspect myself of thinking as the rest of the world think in this matter—except in the case of Rachel Verinder. The self-dependence in her character, was one of its virtues in my estimation. (184.108.40.206)
Most women, according to Mr. Bruff, should allow themselves to be guided by outside advice, while it's good for men, in general, to think for themselves. He's willing to make an exception for Rachel, though, because he knows that she's intelligent and sensible enough to make good decisions, even without asking advice.
So where does that leave us? Most characters in the novel seem to hold the general view that women need guidance from men. But then, Mr. Bruff makes exceptions for women like Lady Verinder and Rachel who don't actually need guidance. It seems that Wilkie Collins might be making fun of the commonly held, sexist belief that women need men's advice to make important decisions. Everyone keeps saying that women need help, but in practice, none of the main female characters actually do.