Pearl's behavior is very different, although I don't know if I would say "unnatural." This is because she is a symbol as much as she is a person, the living incarnation of the Scarlet Letter. Part of her behavior might be attributed to "nurture" --- she is always treated as different and perhaps she is changed by that experience. But she clearly IS the letter, so much so that when her mother takes off the letter in the forest, Pearl refuses to come near her. Factually this might be because she has never seen her mother without it and she actually does not recognize her without it; symbolically she and the letter are so much a part of her mother, that one does not exist without the other.
The most unnatural thing that happens in the story takes place when Dimmesdale "ownes" her at the end of the story and she becomes a changed child. Again, acceptance could make a real difference in her life, but this is a bit much. However, for Pearl as symbol, it's perfect.