he believes is merely the son of Baba's closest friend.

In chapter 18, Amir learns that Hassan is his half-brother, which changes his entire perception of his childhood. Looking back, Baba's actions do reflect the secret that he carried, such as him having paid for the expensive surgery to fix Hassan's lip and his emotional devastation when Ali and Hassan left the house.

After growing up believing he had nothing in common with his father, it occurs to Amir that they were similar in a monumental way—having both betrayed their best friends. Not only did Amir allow Hassan to be raped in order to obtain the kite from Assef; in order to please Baba by winning the kite, he betrays another family member. Similarly, Baba accuses Hassan of stealing, which deeply offends Ali.

At first, it seems like Baba, Amir's father, is acting in an altruistic manner by allowing Hassan and his father Ali to live as his servants. Amir notes that Baba is very loving towards Hassan, and attributes that love to Baba's own giving nature. However, it turns out that Hassan is actually Baba's child, not Ali's; Ali is sterile and Baba fathered Hassan to give Ali the son he always wanted.

Hassan’s not going anywhere, he’d barked. He’s staying right here with us, where he belongs. This is his home and we’re his family.(Hosseini, The Kite Runner, Google Books)

Amir does not find this out until years after the others have died; he realizes that the harsh love shown by Baba towards him was designed to strengthen him and make him sympathetic towards Hassan and his people, since they are all more closely related than he ever knew. He also realizes that the love Baba shows to Hassan is from a true father towards a son, although Baba is never able to truly tell Hassan how much he means to him.

Baba has a very close relationship with Hassan; he treats him like he's his own son. There's a good reason for this: he is his son. Amir doesn't find out about this until later on, but when he does, it explains everything. Growing up, Amir was always jealous of the attention that his father lavished on Hassan. So when Hassan was raped by Assef, he seized the opportunity to have him sent away. This way, Amir hoped to take what he believed to be his rightful place in Baba's affections.

After Amir discovers the truth about Hassan's parentage, he realizes that he has to make amends for the appalling way he treated him when they were young. This involves traveling back to Afghanistan to adopt Sohrab, Hassan's orphaned son. In symbolic terms, Amir is trying to bring Hassan back through his son, reestablishing the connection he had broken all those years ago when he was instrumental in Hassan's banishment.

Baba has a secret which is not revealed until much later in the novel, but it is obvious that Baba treats both Ali and Hassan as more than just servants. Ali has been with Baba since they were children, and they were playmates much in the same way as Hassan and Amir. Baba treats Hassan almost as his own child, and we discover later that Hassan actually is Baba's son by Ali's wife. It explains why Baba admired Hassan and gave him such nice gifts, presenting him with the operation to rid him of his hairlip. When Amir suggests that Ali and Hassan need to be replaced, Baba explodes, telling his son that they are members of the family. Things change between Hassan and Baba only after Hassan is falsely accused of the theft of Amir's birthday gifts. Hassan refuses to defend himself, admitting that he was the thief (and probably understanding that it was Amir who had planted them). Whether Baba knew this is unknown, but he was unable to keep Ali from leaving the household, and Baba never saw his son, Hassan, again.