Which character in the book is morally ambiguous? Why?

Carl Linstrum, the son of Alexandra's neighbors, is the only character I'd describe as ambiguous. He and Alexandra form a great friendship, and by the novel's end decide to get married. His family moves away because of the drought, but eventually Carl comes back. He's a loner; he's thoughtful; he's very smart; and at times he's rather depressing to be around. But he's also very in tune with the people around him. Carl is neither all good or all evil (he isn't at all evil); he just simply "is."

I based this answer on the fact that he's probably the only character in the novel (besides Alexandra) who doesn't do something to hurt another character. I wouldn't consider any of the main characters to except Alexandra (all good) to be innately evil or excessively good. You could use the word ambiguous to describe most any of them and make it work.