I find Atticus Finch the most unchanging character in all of To Kill a Mockingbird. His moral character is unwavering, and he never backs down to a challenge. He is the conscience of Maycomb from beginning to end, and everyone--from his children to his neighbors--seek him for guidance. Despite the town's disapproval of Atticus taking on the Tom Robinson case, he never considers backing down from his obligation to defend Tom. He risks his life defending Tom from the lynch mob at the jail, and the Negroes of Maycomb love him for his determined courtroom defense of Tom, even after the jury brings back a guilty verdict. Threats are made against him by Bob Ewell afterward, but Atticus tries to calm his family's fears and hopes that all will be well instead of taking action against Bob. The father in Atticus always has time for his children, and he never changes his expectations of them from beginning to end.
Only twice do I see faulty reasoning in Atticus' thinking. One comes when he claims that the Ku Klux Klan is no longer in existence around Maycomb. It is historically obvious that the Klan never went away during the 1930s, especially in rural Alabama. It may have been that he was only trying to ease his family's worries about the upcoming trial. Atticus also seems confused following the death of Bob Ewell when he thinks that Jem has killed Bob. It takes some serious explaining of the actual facts from Heck Tate to convince Atticus that it was actually Boo Radley who killed Bob. But aside from these two minor flaws, Atticus is the same Atticus throughout To Kill a Mockingbird.