How does Hassan's silent sacrificial love make him the true hero of the book?
In more recent literature, heroes don't have to be admired by an entire culture. Their actions and decisions reflect personal courage. Their conflicts involve the moral struggles that exist in everyday life. I believe Hassan fulfills this definition of a hero.
The son of Ali, Hassan is a servant at Baba's house, and he is fiercely loyal to Amir. The two young men are not ethnically equal, as Hassan is a Hazara and Amir is a Pashtun. Assef, the boy who eventually rapes Hassan, earlier wants to persecute him just for being a Hazara boy and accuses Amir of betrayal due to his friendship with Hassan. Amir is angered by the bigoted remarks, but he's unable to respond to them in any way. Amir doesn't treat Hassan as a person, even though Amir is close to his servant and playmate. This is seen when Amir takes advantage of Hassan's illiteracy and naivete.
When Hassan is raped, Amir runs away rather than to stay and help Hassan. It's more important to Amir to be able to show his father the blue kite, hoping to raise his father's respect for him by winning the kite runner contest.
Hassan knows Amir knew what happened to him, but he never brings it up. At this point, Hassan displays courage in his decision not to mention Amir's betrayal of him. He keeps the tragedy to himself, knowing that it would cause trouble for Amir, especially with his father. He upholds his loyalty to Amir even though Amir betrays him in the worst way. And remember, Hassan is able to perceive things about Amir that even Amir is not aware of. Hassan is able to perceive Amir's feelings of guilt about the rape and decides to protect his friend and master. Hassan sacrifices himself in order to save his friendship with Amir.
When Amir tells Baba that Hassan stole from them, Hassan apologizes as if he actually committed the crime. Again, Hassan can sense how much Baba's respect and love means to Amir, so he sacrifices himself again to give Amir what he needs from his father.