Hassan is Amir's half brother. Hassan's father, Ali, was like a brother to Baba so he lived with Baba. However, Baba impreganated Ali's wife and had Hassan who became good friends with Amir.
Amir is selfish and Hassan is humble and loyal to Amir. Hassan is a Hazara, Amir is a pashtun. Amir is granted to right to get an education, Hassan is not.
Actually, they are half brothers. Amir had found out that when Rahim Khan had hinted that Ali was not Hassan's father but Baba was (which explains why Baba had a closer relationship to Hassan and Amir).
Rahim Khan tells Amir that Hassan is his half-brother and that they share the same father, Baba. He also informs Amir about the terrible events that occurred to Hassan and his wife, and asks him to come back to Afghanistan to help rescue Hassan's son, Sohrab.
Hassan did not do anything to Assef. Assef was racist towards the fact the Hassan was a hazara and ended up raping him on the last day of the kite running tournament after Hassan had refused to give up the last kite which he had run for Amir.
Amir frames Hassan for stealing money and a watch, knowing that Hassan will take the blame to protect him. Amir stands by and does nothing to stop it, thus indirectly getting both Ali and Hassan in trouble.
if he had not seen him come out of his wifes womb himself
Amir threw pomegranates at Hassan out of frustration and jealousy stemming from their complex relationship and his own insecurities about his father's affection for Hassan. This act was an expression of Amir's conflicting emotions and desire to hurt Hassan in the heat of the moment.
Amir and Hassan, the sultans of Kabul
In the letter that Hassan wrote to Amir in "The Kite Runner," Hassan revealed that he forgave Amir for their past misunderstandings and cherished their childhood memories together. He also informed Amir about his life in Kabul and expressed his hope to reunite with him someday.
Hassan and Ali give Amir a leather-bound notebook for his birthday.
Hassan's first word is Amir. Amir does not reveal his name till this point, which shows his underlying guilt because Hassan loves him so much and Amir's betrayal.
Amir put his watch and some money in Hassan's mattress. Hassan found these items and told Ali about how he got raped while Amir hid and watched, so they left Kabul.