One of the first descriptions of someone as brave comes from Scout herself as she talks about Atticus and the way that he treated some people. She is particularly impressed by the way he treats Mrs. Dubose who is so mean as to be frightening to her and Jem. After seeing Atticus treat her so kindly and speak to her in a way that softens all the meanness that Jem and Scout saw, Scout describes Atticus as "the bravest man that ever lived."
Another example comes when Jem and Scout are forced to read to Mrs. Dubose and they start to understand that something is wrong with her. After her death, Atticus explains that she was addicted and that watching her fight against that addiction proved to him that "she was the bravest person [he] ever knew."
Later in the story, Bob Ewell is described as brave but in a very different way. After his attack on the children, he is described by Mr. Tate as a "low-down skunk with enough liquor in him to make him brave enough to kill children. He’d never have met you face to face." Even though he was a coward, he could drink enough to make him forget all that and be brave in a rather terrible way.