For Atticus courage is to do the right thing, no matter how hard it is, even if you will lose or suffer greatly. There are two examples of this in the book. Mrs. Dubose, even though she could be nasty at times, was an example of great courage. She was a morphine addict who cleaned herself up before she died. She suffered in the process, but this did not hinder her. Amazing perseverance and courage. Here is the quote:
"I wanted you to see what real courage is, instead of getting the idea that courage is a man with a gun in his hand. It's when you know you're licked before you begin but you begin anyway and you see it through no matter what. You rarely win, but sometimes you do. Mrs. Dubose won, all ninety-eight pounds of her. According to her views, she died beholden to nothing and nobody. She was the bravest person I ever knew."
The second character that embodies this is clearly Atticus. He defends Tom Robinson no matter what. He will do so because it is right. He knows that he will lose (he even confesses this to his children), but he will try his best. Even when there was a mob with guns, Atticus stood firm. This is courage in the book. Atticus wants his children to learn this important lesson.