If Santiago is the main character in The Alchemist, who is the foil and why?

Santiago is the main character, or the protagonist, in as much as he is in the story more than any other character and because it his story that we follow. We travel alongside him and watch him develop into a more mature, enlightened individual.
A foil character, who may also be at the same time a protagonist, is a character who is in one important way, at least, very different from the, or another, protagonist. The function of the foil character is to emphasize by contrast the opposite trait in the, or another, protagonist. For example, Eddard Stark in A Game of Thrones might be considered a foil character for Cersei Lannister because his honesty and honor emphasizes by contrast her dishonesty and lack of honor. In the same book, Arya and Sansa Stark are also at once foil characters and protagonists. They are foil characters for one another because the stereotypically boyish behavior of the former is, if you'll forgive the pun, in stark contrast to the stereotypically very girlish behavior of the latter.
In The Alchemist, the Englishman certainly demonstrates attributes which serve to emphasize, by contrast, the opposite traits in Santiago. While Santiago seeks knowledge for some kind of spiritual awakening, the Englishman seeks knowledge for the altogether less noble goal of acquiring riches through the art of alchemy. Santiago also has an emotional intelligence and knows when to follow his heart, in contrast to the Englishman, who buries his head in his books and never learns to look outside of them. In these ways, the Englishman can certainly be considered a foil character to Santiago, the protagonist.