I have included the link for character analysis from enotes. This link will give you detailed information about both characters. Here is a short summary:
Amir is an intelligent and affectionate boy who wants his father's approval and notice. He is ethical and wants to "do right" in life, but his jealousy of Hassan's relationship with his dad leads him to make a bad decision he will always regret.
Hassan is the son of one of the Baba's servants. Illiterate and guillable, he is a happy, athletic, and trustworthy child. He is easily drawn in by Amir and truly believes that Amir is always honest with him. This sincerity makes Amir feel more shame about his betrayal.
I disagree with sullymonster's assesment of Hassan as a gullible character. He is extremely loyal to Amir, but there are instances in the novel where Amir admits that Hassan has an uncanny ability for illuminating Amir's unfair treatment of him. There is a passage in chapters 5-6 where Amir states that when he tests Hassan's loyalty, Hassan tests his integrity. As Amir's servant, Hassan cannot express fully his displeasure about the way Amir treats him, but he makes his point clear in subtle enough ways where Amir feels regretful for his behavior.
Although Hassan is illiterate, he is also very clever and intelligent. This is evident especially in the first four chapters when Hassan listens to stories that Amir reads to him. He analyzes the narratives, asks questions about what he doesn't understand, and is sincerely interested in the educational benefit of learning from Amir. If there is an instant where something Amir tells him doesn't make sense, Hassan questions the ambiguity. So, gullible is not the word I'd use to describe Hassan. Hope this helps.