In The Kite Runner, Amir and Hassan are close friends who come from different classes. As children, Hassan lives in Amir's family's house—his father is a family servant. However, he shares a deep friendship with Amir. The two are playmates and run around playing as long as Hassan does not have any chores or work to do. Hassan has a deeper devotion to Amir than Amir does to him, in large part because Amir is somewhat spoiled as a child.
When Hassan is attacked and raped, Amir witnesses the event but does nothing to stop it or intervene, nor does he tell anyone after the fact. Hassan knows this, and their friendship is forever changed by the event—as they no longer play together and they speak much less frequently. There seems to be a wall between them. As Amir grows, he feels great remorse for his actions towards Hassan, and they drive him to seek him out later in life.
As boys, Amir and Hassan were separated by social class, but they were united by the fact that they were young and the only two children in Baba's house. They played as children play, exploring their surroundings and sharing adventures. Hassan loves Amir without limits or conditions. Amir, however, is inconsistent in his treatment of Hassan; sometimes he treats Hassan as his friend, but frequently he abuses Hassan, lording over him his superior social class, education, and position as Baba's son. Hassan is, after all, a Hazara and a servant in the house. Hassan suffers the brunt of Amir's insecurities without complaint. The more Amir mistreats him, the greater Hassan's love and loyalty. It is Hassan, very small in stature and armed only with a slingshot, who saves Amir from the neighborhood bullies, led by the cruel Assef.
Their relationship in Kabul ends when Amir betrays Hassan; first he stands by to watch Assef and his gang assault and rape Hassan; then, because he cannot stand his guilt, he frames Hassan to make it appear he is a thief, forcing him out of Baba's home. These acts of cowardice haunt Amir for the rest of his life, until he atones for them years later. Despite Amir's betrayal, however, Hassan's love for his friend never wavers. It is his letter, years after their parting, that puts Amir on the road to redemption and self-respect. Thus, Hassan saves Amir twice, once as a boy and years later, even after his own death.