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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.
Amir feels immense guilt and remorse upon discovering that Hassan is his half-brother, as he realizes that Baba had kept this crucial truth from him. This revelation deepens Amir's sense of betrayal towards both Hassan and Baba, and raises questions about his own privilege and cowardice in the way he treated Hassan. The revelation ultimately prompts Amir to seek redemption and make amends for his past actions.
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).
Hassan retrieves the blue kite for Amir during the kite running tournament in Kabul. However, the betrayal and assault by Assef, backed by his friends, leads to Hassan's traumatic experience as he is physically and emotionally hurt. This event significantly impacts Hassan and Amir's friendship, ultimately leading to their separation.
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.
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
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.
Baba shocks Amir by letting Hassan and Ali stay at the house, as Amir had put the wristwatch and Afghani bills under Hassan's mattress so that when Baba did find out about Hassan's stealing (a trait that Baba once told Amir he would never forgive in a person), Baba would want to push them out. Instead of Baba kicking them out, he forgave Hassan for "stealing" (he only admits stealing from Amir to protect him), and begs them to stay. Hassan and Ali leave the next morning and Baba weeps when they leave.
Amir feels jealousy towards Hassan when Baba shows favoritism towards Hassan, when Hassan excels at kite running, and when Hassan receives attention and affection from Baba. These instances highlight Amir's deep-seated feelings of inadequacy and envy towards Hassan despite their close relationship.