Hassan's cleft lip adds to his overall "underdog" persona. Not only is he Baba's illegitimate child, he also has a disfigured face. Despite his disfigurement, Baba treats him well, and Amir is even jealous of him. The harelip adds another reason why Amir should NOT be jealous of Hassan, and it shows the reader how Hassan's good heart and loyalty can trump his appearance. Despite the fact that Hassan is physically unattractive, he is smart and good at everything he does.