This is an excellent question as clearly the relationship between Amir and Hassan and what happens to it is a key theme of the novel that is only really resolved at the very end of this gripping tale. It is clear that, in their childhood, Amir and Hassan have an unequal relationship. Hassan is officially Amir's servant, and yet it is clear that they have a closer friendship. However, at the same time, this "friendship" is one that Amir feels he can use to bully and to be cruel to Hassan as and when the occasion suits him. One particularly key moment in Chapter 5, that foreshadows the rape of Hassan that Amir does nothing to stop, is when Assef bullies Hassan and Amir and Assef verbally attacks Amir for having Hassan as a friend:
But he's not my friend! I almost blurted. He's my servant! Had I just thought that? Of course I hadn't. I treated Hassan well, just like a friend, better even, more like a brother. But if so, then why, when Baba's friends came to visit with their kids, didn't I ever include Hassan in our games? Why did I play with Hassan only when non one else was around?
This quote is key for a number of reasons. In it we gain an insight into Amir's mind as he is forced to admit the fact that he doesn't treat Hassan like a friend, even though at times (and highly ironically) he says he treats him like "a brother." It captures the ambiguity in Amir's relationship with Hassan and how he only treats him as a friend when it suits him and there is something he can gain from it. In their relationship, however, Amir always plays the part of the master and Hassan is always the servant.