Courage is shown in the character of Hassan in so many ways, though it is definitely not shown in the character of Assef. Note the way that as children, Hassan is ready to challenge Assef, even though he is outnumbered, by holding a slingshot up towards him and threatening him with it. Even though Amir recognises Hassan is scared, Hassan still continues to face Assef down, and to successfully make him go away with the following threat:
But perhaps you didn't notice that I'm the one holding the slingshot. If you make a move, they'll have to change your nickname from Assef "the Ear Eater" to "One-Eyed Assef," because I have this rock pointed at your left eye.
The bravery and courage that it must have taken for Hassan to make this stand must have been incredible, and it certainly acts as a rebuke for Amir, who feels that Hassan demonstrates the kind of courage that Baba would expect of Amir himself. Hassan later on demonstrates courage by protecting Baba's house, even when it leads to his and his wife's death.
Assef, by contrast, as an adolescent only walks around with two friends for protection and seems to avoid confrontations where he would be outnumbered. Even though, as an adult, he faces Amir by himself, and deliberately tells his guards to not intervene in any way, he still insists on having the advantage with his brass knuckles. There is little of courage in this psychopathic character. Hassan is definitely the character who is much more courageous.