Most of what we know about Beryl we find out from other people after the Baskerville mystery has been resolved. We know she's Stapleton's wife, rather than his sister, as he originally claimed. We know she initially went along with all of his plans—even going so far as to lay a path through the Grimpen Mire to a safe island so that he'd have a place to keep the Hound. We know that Stapleton tried to use her to lure Sir Henry so that he could sic the Hound on him one evening (though Stapleton's own jealousy got in the way of this plan).
But we also know that, at some point, Beryl does try to sabotage Stapleton's plans—though, importantly, never in a way that might actually harm him or his reputation. She's the one who cut out words from the Times to send that warning letter to Sir Henry in London. She also tries to warn Sir Henry in person to leave Baskerville Hall (without realizing that she's actually talking to Watson).
So Beryl comes across as a highly ambiguous figure in the novel: she tries to stop Sir Henry's murder, but she never willingly reveals Stapleton's secret and she's also her husband's willing accomplice through a lot of his earlier crimes.
Holmes later speculates that "Stapleton exercised an influence over [Beryl] which may have been love or may have been fear, or very possibly both, since they are by no means incompatible emotions" (15.23). These ties of love and fear were enough to keep her loyal to Stapleton from the time that they were married in South America to the time that they posed as a couple running a school in Yorkshire under the name of Vandeleur to this final fraud as the Stapletons in Dartmoor.
What really turns Beryl against her husband is her discovery that he's also hitting on Laura Lyons. As Holmes explains later, on the night of Stapleton's attempted murder of Sir Henry, Beryl and Stapleton argue over his plans, and "a furious scene followed, in which [Stapleton] showed her for the first time that she had a rival in his love" (15.23).
At that moment, Beryl turns against Stapleton completely. That's why he leaves her tied up in the bedroom as he eats dinner with Sir Henry. And that's why she leads Holmes and Watson to his hideaway in the middle of the Grimpen Mire. The English playwright William Congreve was right: "Heav'n has no rage, like love to hatred turn'd, / Nor Hell a fury, like a woman scorn'd" (source).
Say it Ain't So, Conan Doyle
One last note on Beryl: there's some racism in Conan Doyle's portrayal of her spirited jealousy and powerful hatred for her husband. When Holmes sums up Beryl's attitude toward Stapleton on that final night, he warns Watson that, "if we had not been there, his doom would none the less have been sealed. A woman of Spanish blood does not condone such an injury so lightly" (15.23). What makes her any different from the totally English Laura Lyons, the other woman in this novel who also has a thing for Stapleton and eventually turns on him? Seems like some old stereotypes at work here.
The larger point of Conan Doyle's characterization seems to be that women in general let their emotions overcome their judgment—an idea of women that may fly in 1901, but that seems pretty old-fashioned and ridiculous now.
Intended to incarnate ill will and malice, Stapleton is conflated at various points with the lecherous libertine Hugo, whom he resembles. Stapleton is a black-hearted, violent villain hidden beneath a benign, bookish surface.
If Hugo operates as a kind of Doppelganger for his entomologist heir, then the convict offers an interesting parallel as well. Serving mainly as a red herring in the mysterious death of Sir Charles Baskerville, the convict also operates as a foil for the real culprit, Stapleton. Personifying "peculiar ferocity," "wonton brutality," and even dubious sanity, the convict is shown to be a pathetic, animalistic figure on whom the detectives ultimately take pity. Not so with Stapleton, a man with a "murderous heart," and a wolf in sheep's clothing.
Stapleton is a worthy adversary because of his birthright. If the convict is a simple murderer, he is also simply born, related by blood to the Baskerville's domestic help. Thus, the convict is part of a lower class than Holmes, and therefore is not a worthy adversary. Stapleton, however, is an intellectual, and when his evil side comes out, his hidden nobility comes out as well. Once Holmes is handling an educated and noble rival, he begins to take things much more seriously. In this sense, Stapleton's character adds to the strong classist themes imbedded in this book.
More characters from Hound of the Baskervilles