The character of Pearle is a complex one though at first that may be hard to see through the turmoil and suffering Hawthorne surrounds her with. And, indeed, one of her main functions is to embody the turmoil and suffering that results from the twin evils of adultery and the extraordinary punishment incited by adultery. Pearle's function in Hawthorne's The Scarlet Letter is to illuminate the psychological aspects of the novel and to underscore Hawthorne's thematic concerns.
First, Pearle illuminates psychological aspects by being a constant reminder of the act of adultery that led to Hester's suffering and public shaming. Pearle also is symbolic of the adulterous act itself in that she is both Hester's joy and prize, her "pearl of great price," and her torment, which is given because Hester is horrified by Pearl's early identification with the scarlet letter and by the imp that seems to posses Pearle. In correlation with this, Pearle is also the living embodiment of that scarlet letter and depicts the isolation resulting from the adultery: isolation for Dimmesdale, Hester and Pearle.
Further, Hester is unable to instruct Pearle in right behavior; she has tried but failed. In this Pearle functions to represent the painful consequences of unruly, wild, passionate, unnatural deportment by being the unnatural product of adultery. In addition, Pearle helps move the plot along by giving Hester added aggravation to her inner psychological deterioration.
Second, Pearle underscores Hawthorne's thematic concerns by making his points for him. Those points being that Puritan intolerance and extraordinary punishment destroy innocent children's minds as surely as adultery destroys their hope of a natural and happy life and that such intolerance and punishment subverts Christ's doctrine of love. While using Pearle to protest harsh and unloving punishment on one hand, Hawthorne paints with the other hand a representation of an allusion to the apostolic epistle, The First Letter of John, that enjoins us to sin not, but then explains what to do when we do sin, thereby sealing his protest against Puritanism's unloving, unforgiving punishments.