The main character of Percy Jackson is, guess who, Percy Jackson! I think you have no clue what a protagonist is becuase Krazychic is right. so idk why you are correcting him. Percy is played by the horrible Logan Lerman. Antagonist is what subject17 said.
if you mean protag then percy (wow like thats not obvious) but if you meant antagonist then it is kronos and or ares technically its kronos but you could argue if your talking about exclusively the first book that is ares as well