There are many characters who arguably have heroic qualities. For example, Mrs. Dubose shows great heroic spirit in her efforts to wean herself of morphine. However, overall, the hero of this novel for me is Atticus Finch.
Atticus Finch is part of one of Maycomb County's oldest and most important families, yet he does not use this position to gain any special privileges. Unlike others in his County, Atticus is an example of tolerance and understanding. He has earned the respect of his fellow townspeople through his dedication to his job and his character, even when they don't approve of his actions. We can see this when they do not approve of Atticus' decision to defend Tom Robinson to the best of his ability.
As a father, Atticus seems to be able to strike the perfect balance between allowing his children freedom to explore and experiment but at the same time being a disciplinarian and making it very clear to them when they have overstepped the line and how their actions effect others. We see this for example when he makes Jem read to Mrs Dubose every day. But interestingly, his punishments always teach his children valuable lessons, such as the true nature of courage, which, as Atticus tells his children, is "when you know your licked before you begin but you see it through no matter what". Of course, Mrs. Dubose's courage foreshadows that of Atticus in defending Tom Robinson, in spite of the increasing hostility he faces. It is this commitment and dedication to the truth and to his ideals that makes him the hero of the novel.
Within the definition of heroic as involving recourse to bold, daring, or extreme measures, Boo Radley may be considered a hero. While his presence is subtle throughout most of the novel, it is felt, nevertheless. For, he becomes a central figure in the imagination of the children who play "Boo Radley games." And, despite Boo's reputation, he is a gentle person who leaves secret gifts in a tree, he mends Jem's torn pants when the boy is frightened after a dare, and he places a blanket over the shoulders of Scout after a fire. In a way, Boo is like a spectator in the lives of Scout, Jem, and Dill.
However, at the end of the novel when Jem and Scout are threatened by Bob Ewell, the reclusive Boo who has spent fifteen years hidden in his house, summons in himself the courage to take the extreme measure of leaving the safety of his house and heroically defend the children as a father would against a vicious man who threatens his family.
That this act requires heroism is evident in the movements of Boo after the incident. In Chapter 31 of "To Kill a Mockingbird," Boo
shuffled to his feet...Every move he made was uncertain, as if he were not sure his hands and feet could make proper contact with the things he touched. He coughed his dreadful railing cough and was so shaken, he had to sit down again.
He is so shaken that Scout has to lead him out of a room and he, child-like, asks her, "Will you take me home?" So, after the children are safe, Boo Radley is no longer heroic, but for a monumental time in the lives of Jem and Scout, he is their hero.