A lot of change happens throughout The Kite Runner, by Khaled Hosseini, particularly to the protagonist, Amir. His childhood friend and half-brother, Hassan, experiences few changes, but certainly both are forced to adapt to many changing circumstances in their lives.
Amir's changes are many. He grows up as a young child who longs for his father's love and approval but never feels as if he gets it from Baba. He is inexplicably jealous of Hassan's relationship with Baba, and of course later he discovers his true relationship with Hassan which explains everything. Amir's and Hassan's friendship is distinctly lopsided, with Hassan doing all the compromising and routinely accepting the abuse Amir gives him. Amir proves that he is not a friend to Hassan when he sees Hassan being horribly and violently abused by the perverted bully Assef but does nothing to stop it. He says:
I actually aspired to cowardice, because the alternative, the real reason I was running, was that Assef was right: Nothing was free in this world. Maybe Hassan was the price I had to pay, the lamb I had to slay, to win Baba.
He feels guilty and ashamed of himself, but instead of dealing with those feelings openly, he harbors them and then uses the first opportunity he can find to get rid of what he sees as the source of his guilt--Hassan.
Soon Amir and Baba are force to leave Kabul and they settle in America. As Amir matures, he begins to see his father differently and accepts him for who he is and how he raised Amir; his feelings toward Hassan also change, though his guilt and shame are a constant presence in his life. When he is offered a chance to redeem himself and atone for his past sins, he does so, though unwillingly, at first. In the end, he makes amends for both himself and Baba as he raises Hassan's son with true love and respect.
Hassan is cheerful, loyal, and loving at the beginning of the novel, and he is the same at the end of his life; however, these are the very qualities which cause him pain. He loves Amir so much that he allows himself to be abused by him in the name of friendship. He always says,
“For you, a thousand times over”
His attempt to please and serve Amir is what puts him in the position to be abused by Assef, and his loyalty to Baba is what eventually gets him killed. He was living a peaceful life in a remote village when Rahim Khan asks Hassan to come back to Kabul to help him maintain Baba and Amir's house, and Hassan is killed while doing so. He is not perfect, and he has to work through his anger and disappointment with both his mother and with Amir, but it does not take long for him to forgive and love them unconditionally.
His last letter to Amir is full of love and respect, despite Amir's treatment of him, demonstrating that Hassan is the same character at the end of his life as he was from the beginning.
Hassan does not undergo as many changes as Amir because he does not have as many things about him that need changing.