It is almost blasphemous to suggest that Amir is brave or courageous after he watches Assef rape Hassan and doesn't do anything about it. It is even harder to suggest it when he tries to frame Hassan for stealing in order to get him sent away.
The bravery and courage that Amir demonstrates come later in the story, after he has been unable to face his horrific actions as a boy. It is only when Rahim Kahn, who was aware of what happened, calls and tells him that he has a chance to redeem himself that he begins to face his fears.
He faces them with an impressive amount of courage when the time comes. When he has to go and meet with Assef and Assef is beating him so badly that he begins to assume that he will die, he begins to laugh at the relief he feels at finally standing up to him. He knew that his "body was broken... but [he] felt healed."
And he fights desperately to save Sohrab both from Assef and later from himself. He even has the courage to bring him home to his wife and to try and build a family around him despite having a clear understanding of how difficult that will be.