There perhaps isn't so much a clear protagonist in the story of David Swan. In a way Fortune serves as the driving force for good throughout the narrative.
David Swan himself is a bit reckless and naive, carelessly falling asleep next to a pool without thinking of possible dangers that might befall him. This decision also displays a bit of laziness. He is on his way to Boston where he will have a job working at grocery store. However, the only reason he was able to secure this job is because his uncle is the store owner. This is the first instance in which fate favors David Swan.
Fortune favors him again when the merchant and his wife look upon David and even foolishly desire that he replace their son who has just died. This would make David Swan the heir to immense wealth. However, David is oblivious and continues sleeping. Again a young woman stumbles upon him. Had he awoke at this moment he would have become her father's clerk and a wealthy, distinguished man.
Finally when two thieves come to prey upon David, a man and his dog appear scaring them away and ensuring the snoozing Mr. Swan's safety.
This Fortune, which is compared to providence and divine aid in the last paragraph, lays at David Swan's feet countless golden opportunities. As Hawthorne notes, humanity shares David Swan's lack of foresight an neglects to make use of these opportunities.