What is the evolution of Sylvia's character in "The White Heron"?
At the beginning of the story, she is a shy, rather uncertain girl who is easily impressed by others, and willing to do things to gain their approval. She is also rather naive and innocent, with very little exposure to human frailty, greed, vice or cruelty. She has led a rather sheltered life that has protected her from having to deal with the reality that humans can be bad people sometimes.
All of these traits come out when she first meets the rather mysterious and handsome stranger in her area. She is a bit intimidated by him, wowed by his newness and ways, and very eager to please and help him. She wants to impress him and make a good impression; her impressibility comes out, and her uncertainty. However, as she learns more about what the man does, she internalizes it and realizes that she is going to have to make a big decision.
It is when he wants to find the heron that we see some real changes develop in her. In her final decision to protect the heron over the man, we see her more confident, assertive and willing to forego the approval of someone new and exciting in order preserve someting more sacred and beautiful. Her innocence and naivety have been replaced with the knowledged that there are people out there who are willing to destroy nature and make bad decisions. She shows great strength of character and maturiy when she decides to protect what she knows, in her heart, to be more important. Her uncertainty is gone; she is certain, a bit more wise, and confident in her principles.
I hope that helped a bit; good luck!