I feel as though the cleft lip is a very important symbol.
In the beginning of the novel Hassan's cleft lip symbolized his poverty and low status. With the split lip Hassan was capable of standing up to Assef and being strong. Amir at the beginning did not have a split lip and could not stand up against Assef or save Hassan from being raped. Later on in the novel Amir fights Assef and ends up with a split lip that proves he has then became emerged with Hassan and learns he can now stand up and care the way Hassan did.