Santiago is the protagonist of Paulo Coelho's The Alchemist, though he is referred to as such only once in the text. He is a Spanish shepherd boy who goes on a quest to find treasure near the Egyptian pyramids. He undergoes great personal transformation as he makes his journey, meeting strange characters who help or distract him along the way.
Interestingly, throughout the majority of the narrative, Coelho calls him "the boy," referring to his proper name, Santiago, only once. As mentioned in the previous answer, this could fit in with the novel's status as a coming-of-age narrative, showing how "the boy" has not yet come into his own personhood. However, it is strange how Santiago is never called so again, even after he achieves his maturity.
Another reason for this authorial choice could be that it is Coelho's way of making Santiago's story seem more universal. Santiago is not merely an individual; he is an avatar for all people who strive to follow their dreams.