The main character is Percy, no doubt. However, in the context of this fanfiction, new characters might be introduced. Maybe there's a mysterious demigod who has been silently observing the gods' actions and joins Percy when he snaps. But the old gang like Percy, Annabeth, and Grover are likely to be the mainstay. They each bring their own skills and relationships to the story. Annabeth's wisdom, Grover's connection to nature, and Percy's courage all play important roles in whatever they plan to do against the gods.
Obviously, Percy would be one of the main characters. He is the one who snaps at the gods, so the whole story likely revolves around him. Annabeth could also be a main character as she is Percy's close friend and would likely be involved in his plans or at least support him in some way. Grover might be there too, as he has always been by Percy's side in many adventures.