Assef is a villain from Khaled Hosseini's novel, The Kite Runner. A violent sociopath, he is driven to assert dominance over others in ways that include violating, harming and killing other people. He is also a bigot who admires Adolf Hitler.
As a child, he rapes Hasan, a member of the Hazara minority whom he looks down on. As an adult, he becomes a high-ranking member of the Taliban, and takes small children from orphanages and sexually abuses them.
As a child, Assef is a bully who terrorizes other children, especially those of the Hazara minority. He comes from a wealthy family, with a politically connected father who gets him out of trouble. As an adult, he uses those connections to become a high-ranking member of the Taliban - not out of any particular religious devotion, but simply as a way to have power over other people and commit acts of cruelty and violence.
He takes sadistic pleasure in hurting people, and is especially keen on sexual violence, raping those weaker than him to keep them under his control. He is also a bigot who sees the Hazara minority as inferior, and wants to exterminate them, much as his hero Adolf Hitler had tried to exterminate all the Jews in Germany.
Assef first appears with his companions to taunt and bully Amir and Hassan, who are walking down the street. He often questions their friendship, and mocks Hassan with the idea that "he is really just a servant to Amir".
After Amir and Hassan win a kite-fighting competition, Hassan gets lost and ends up in an alley, where Assef and his friends corner him and demand he give them the kite. When Hassan refuses, Assef decides to "teach him a lesson" - he rapes Hassan while his cronies hold him down. Amir witnesses the whole thing, but instead of helping, he runs away and avoids the guilt by forsaking Hassan's friendship.
One night at a party, Hassan attended the party where he socialized with the adults and had a fun time at the expense of Amir's misery. When Amir and part of his family had to relocate to avoid terrorists, it was revealed that one of Assef's closest friends, Kamal, was raped by Assef.
Decades later, Assef joins the Taliban and is responsible for numerous deaths that had happened in the region as a result of the Taliban's conquest. After Hassan is killed by the Taliban, Assef kidnaps his son, Sohrab, and keeps him as a sex slave.
When Assef first appears, he stones a man to death for no reason. When Amir confronts Assef and demands he release Sohrab, Assef refuses, which results in a physical altercation. Ultimately, Assef loses the fight, and Sohrab throws a ball bearing into his eye, blinding him and giving Sohrab and Amir a chance to escape.