In the 'Monster of Mayport Story', if it's about a sea - like monster terrorizing the town, the main characters might include fishermen who have encountered it. Or if it's a more land - based story, perhaps the local sheriff or a group of brave townsfolk could be the main characters. They would be the ones dealing with the 'monster' whether it's a real threat or just a superstition.
Since I don't know the details of the 'Monster of Mayport Story', I can't say who the main characters are. There could be a hero trying to solve the mystery of the monster, or the monster itself could be considered a main character.