Pearl was fascinated by the scarlet letter and came to associate it strongly with her mother. Pearl is presented in an almost supernatural light throughout the book, often acting more like a spirit than like an earthly child. She is the Romantic child of nature, strongly linked with spiritual purity, despite her wild ways. Just as the letter is a physical symbol of Hester's sin and estrangement, so too is Pearl a living symbol of those ideas.
When Hester briefly considers running away with Dimmesdale, she casts off the letter, but this disturbs Pearl. When Hester beckons Pearl to come to her and Dimmesdale, Pearl begins to "burst into a fit of passion, gesticulating violently" when she sees her mother without the letter on her chest. She only recognizes Hester again when she re-fastens the letter.
In this way, the scarlet letter is as linked to Pearl as it is to Hester. For Hester to toss aside the letter and attempt to escape the consequences of her actions would mean abandoning Pearl—at least, this seems to be a possible interpretation for Pearl's wild reaction. Pearl also washes off Dimmesdale's kiss in the brook. Pearl's violent response against the lovers suggests that their plan to run away together will not solve their problems; it might even be the wrong thing to do. To run away together would only be a continuation of their sin, working against their atonement. Hester must continue to grapple with her guilt, and Dimmesdale must publicly recognize Pearl as his child so that he will no longer live in hypocrisy. Pearl's reaction makes the two of them realize this truth.