The first major confrontation that we see between Amir and Hassan and Assef is one in which Hassan distinguishes himself and Amir shows himself to be weak and not able to stand up for himself and for those he loves.
Amir's voice trembles as he speaks to Assef. His relationship with him is defined by fear, as he recognises the kind of madness that Assef is subject to and the way in which this empowers him to do anything. Note how he responds when Assef takes out his brass knuckles:
I looked in his crazy eyes and saw that he meant it. He really meant to hurt me. Assef raised his fist and came for me.
Whilst Amir's relationship with Assef is defined by fear, Hassan, although afraid of Assef, is able to make a stand and manages to threaten Assef in order to protect his friend and master and stop him harming him. The way that he stands there with a stone in his slingshot pointed at Assef's face demonstrates his bravery and also his courage, which is of course something that Amir by implication shows he does not possess. Hassan is another character that is used as a foil for Amir in this novel, and another character that shows Amir to be wanting in basic moral virtues such as courage and bravery.