I think the most important place to start would be the difference in social status of the two boys. Hassan is Hazara, which makes him of lower class than Amir. There should be a quotation right in the first few chapters about this. For example, when the Hassan uses his slingshot to threaten the boys who are picking on Amir, they make a comment about how Amir is only Hassan's friend when no one else is around for him to play with. They have the same faith, so that could be something similar which they share. I also like the idea of taking faith and talking about the concept of faith in each other. Hassan has faith in Amir and thinks that he can do anything. But Amir has no faith in himself or anyone else, for that matter.
On to personality, a good example from the book would be how Amir is so mean to Hassan after the rape. There are two examples that stick out to me. First of all, when Amir pummels Hassan with pomegranites and Hassan refuses to fight back because he loves Amir so much. Next, the betrayal. Amir really shows his lack of character when he accuses Hassan of stealing his birthday watch and money so that Hassan will be sent away. It is hard to like Amir because he betrays his friend. Whereas, it is impossible not to love Hassan from the moment you meet him in the novel. Amir is extremely selfish while Hassan is almost too kind and caring.
As for experiences, how about the fact that Hassan is raped and that would never happen to Amir because of how important Baba is? Amir leaves Afghanistan and goes to America while Hassan stays behind in the war torn nation. Amir can read and write while Hassan cannot. Both men get married. Hassan has a child while Amir and his wife cannot.