The Scarlett Letter, by Nathaniel Hawthorne, is set in Puritan New England; it is a time of great suspicion and therefore also of great accusation. Sin is condemned in the strongest terms, both from the pulpit and from the Puritans themselves. They are even quicker to pass judgment and condemn others when they are trying to deflect people from seeing their own sins. It is against this backdrop that Hawthorne places Hester Prynne, her former husband, her secret lover, and her daughter.
One of the peripheral characters in this novel is Mistress Hibbins, the governor's sister who is later killed for being a witch. (Hibbins is based on a real woman in Puritan history who was, in fact, condemned and killed for practicing witchery.) She does not appear often in the novel, but when she does she seems to recognize the "hidden sins" of both Hester and Dimmesdale, and several times she invites Hester to join her in the forest (a symbolic place of darkness, evil, and sin).
“Wilt thou go with us tonight? There will be a merry company in the forest; and I well-nigh promised the Black Man that comely Hester Prynne should make one.”
Though she does not play a major role in The Scarlet Letter, Mistress Hibbins is a grim reminder that Puritans do not tolerate overt sins, such as the practice of witchcraft and adultery (though it certainly allows hypocrisy and judgmentalism in seemingly pious people to continue unchecked). Hester tells Pearl she has been visited by the "Black Man" only once, and the letter that she wears is her mark; this connects Hester to the only other woman in the novel who claims to have dealings with the "Black Man" (Satan).
Each time Hester is at a crossroads, she meets Mistress Hibbins, such as at the Governor's mansion and the scaffold. Mistress Hibbins is both a reminder of Hester's sin and her choice of how to deal with that sin. These are two constant thematic and plot elements in the story, which make Mistress Hibbins essential to the novel.