I'm almost afraid to post this, considering how combative this thread is... but:
Does Theresa have any character development going on?
I understand your point about her not changing and I think it's a fine enough idea to have someone who stands by their morals no matter what, but something about her has to change to make her an interesting character. If she doesn't change, doesn't have faults, doesn't fail every now and then, then I don't think she is the kind of character I would like to read about. Isn't the best part of fiction the way you can put your characters under extraordinary stresses that are impossible in the real world, and then watch how they change as a result?
Also, I want to say that I disagree about people not changing because of experiences. Everyone changes because of every experience, even if only in a minute fashion.
If people don't think people change because of the environment, they are close minded individuals who still think the world is flat, the sun rotates around the earth, and that there is such a thing as a free lunch. I'd beg the question, what experience wouldn't change someone. For me this argument is in the truth of the matter and it's real effects. I can give you thousands, even millions of things that change a person in some way. So tell me just a few things that doesn't. I've almost got a degree in Sociology and believe me I've learned more about human nature than I ever wanted to know.