Santiago is the pivotal character in the story; it is around him that both the story line and the main themes are woven. Santiago is first pictured as an aging old man resisting and then defeated by nature. But at the end of the story he emerges as a heroic archetype as new definitions of "success" and "failure" are fomulated.
I suggest you read the essay mentioned in the reference below as it gives greater insight into Santiago's role in both plot and theme.
"Santiago is an old fisherman of undetermined age," and he was the protagonist of this story. He was the main character. He is symbolic for the concept that man should never give up on his dream. He should fight to the end.