When Emma first meets Leon, she feels that she has found a kindred spirit. But we know that Leon is as superficial as Emma. Both are distinguished from the other members of the society because both strive for some feelings that transcend this society. But both are also trapped in their own romantic dreams.
At first Leon is terribly shy and unsure of himself. He is inexperienced in the world of women and love. He has lived too long in a world of sentimental romantic fantasy. Thus when confronted with Emma, he cannot bring himself to tell her of his real feelings. His fears overcome him, and he is afraid of being ridiculous.
After Leon has been to Paris, he gains more confidence in himself. When he returns to Paris, he retains many of the Parisian manners and "airs," giving the impression that he is really the master of any situation. Thus when he meets Emma again after three years' separation, he has acquired a thin veneer of sophistication, but he is still a shallow and weak young man. Even though he begins a love affair with Emma, he realizes himself that he is not the master of the situation. He is unable to act in an aggressive or decisive manner and allows Emma to lead in their relationship.
Furthermore, Leon also serves to illustrate the divergence between Emma's dreams and her reality. She forces Leon to conform to her idealized concept of a lover. She refuses for a long time to face reality, and the contrast between Flaubert's objective description of the weak, fluctuating Leon and Emma's idealized conception of him underlines Emma's predicament.