She is a shy, independent, considerate child. She enjoys being in the country with her grandmother, as opposed to the busy city. She is content with the simple things in life, like the beautiful countryside and her only companion, the cow Mistress Moolly.
She is also very mature, and makes a hard decision in the end to preserve the bird, at the sacrifice of her only human friend in quite some time. She values the beauty and life of the creatures around her over the pleasures of human companionship.
Her maturity and independence stand out. For a girl her age, those two traits are pretty rare. Most often, teenage or preteen girls value sociality, friendship, and peer-approval, and put all else above having those things. But Sylvia is unique; she relishes being alone, is afraid of most human interaction, and makes a tough decision at the end that sacrfices a tender friendship.
Sylvia changes from a shy little girl into a young adult during the course of the story. At the beginning of the story, her greatest pleasure is "to hide herself away among the huckleberry bushes." When the young ornithologist comes, Sylvia is attracted to him, both because he is a young man and because he is offering her a great deal of money. However, during her effort to climb the old pine tree, Sylvia experiences a transformation. At first, Sylvia is described as "a little girl" and "small and silly Sylvia". As she begins her journey up the tree, she is described as a "spark of human spirit". Finally, as she reaches the top of the tree, she is called "a pale star" and stands "triumphant". To show her growth, she can now look down two hawks and her perspective changes to include the ocean and distant farms. She has discovered "it was a vast and awesome world." Because of her journey into maturity, she is able to resist the hunter who only wants to kill the white heron for a "specimen" and enjoy nature for its true beauty.