Helen is a foil to her more measured older sister, Margaret. She's an uninhibited, wild, and self-indulgent version of Margaret, in whom we see romantic and liberal theoretical notions about society, life, and love all allowed to explode in every direction. Unlike Margaret, Helen doesn't think at all about the practical necessities of life, choosing instead to live entirely through her feelings and passions. This leads her to make a lot of mistakes – namely, sleeping with Leonard on a whim because she's taken by his state of utter desperation. Helen shows us what Margaret could be like if she weren't so plagued by things like money and convention…that is, the trappings of the real world.
Helen is also the polar opposite of Henry Wilcox, and their conflict is really the central problem of the book – Margaret is caught between these two extremes of liberalism and conservatism, and the project of the novel is to reconcile the two.