Lucy is the other female British character, and she provides an interesting contrast with Mina, our heroine. She's bubbly where Mina is quiet and thoughtful. She's also much sexier than Mina – three men want to marry her, and she'd be happy to marry them all, whereas Mina seems relatively sexless.
The three Brides of Dracula also act as foils to our heroine, Mina. Jonathan Harker, Mina's fiancé, is weirdly attracted to, and yet repulsed by them, and doesn't want Mina to find out. Later, he considers a comparison between them:
I am alone in the castle with those awful women. Faugh! Mina is a woman, and there is nought in common. They are devils of the Pit! (4.67)
Harker insists that there's nothing in common between Mina and the sexy vampire ladies. Does that mean that he doesn't want to think of Mina in terms of sexual desire, as he (briefly) thought of the Brides of Dracula? Or simply that Mina isn't a blood-sucking she-demon? Either way, we're invited to think of the Brides of Dracula as sexy (but evil) foils to Mina.