Mistress Hibbins first appears just after Hester learns the magistrates are considering taking Pearl away from her. Reverend Dimmesdale speaks on her behalf, leading to Hester being allowed to keep Pearl. As Hester and Pearl leave the governor's house, Mistress Hibbins peeps out a window to invite Hester to come with her to a witches' meeting with "the Black Man" (the devil) in the forest that night. Hester, with a "triumphant smile," declines the invitation, claiming that if Pearl had been taken from her, she'd be happy to join the witches.
The next time Mistress Hibbins appears is during the procession, when she makes comments to Hester that indicate her awareness of Hester and Dimmesdale's trip to the forest as well as their original affair. Her words foreshadow the book's end when she says that the minister has something similar to Hester's scarlet "A," something that he conceals. Moreover, she says that when the Devil sees someone like this, someone who hides his sinfulness, "he hath a way of ordering matters so that the mark shall be disclosed in open day-light to the eyes of all the world!" Mistress Hibbins explains she doesn't need proof to know all of this; she can tell by the way Dimmesdale acts.
Both of these instances show just how perceptive Mistress Hibbins is; she seems to intuitively know the contents of another's heart. The majority of Puritans may be utterly blind to such a possibility, and this is another criticism Hawthorne launches at them. Mistress Hibbins senses Hester's heart rebels against the community, though she seems to conform and accept her punishment with equanimity. She later senses Dimmesdale's guilt, despite his attempts to conceal his sin. It is interesting this supposed witch actually knows more about the human heart than many of the rigid, judgmental Puritans do.