Amir is always beset by fear when he encounters the older bully, Assef. The son of a Pashtun father and German mother, Assef considers himself among the social elite of Kabul. Because of his German heritage, Assef worships Hitler; he also believes, as many Pashtuns do, that the ethnic Hazaras (like Ali and Hassan) are inferior. It sickens him to see Amir seemingly treating Hassan as an equal, and he perceives them as inseparable friends. His own relationship with Hassan is also troubling to Amir, who can never bring himself to call Hassan a true "friend." He wants to tell Assef this, too, but he fears Assef, and in some small way, he does not want to hurt Hassan's feelings.
But he's not my friend! I almost blurted. He's my servant! Had I really thought that? Of course I hadn't. I hadn't.