User blog comment:Athletiger/Featured on the Main Page/@comment-3247762-20120220213706/@comment-3146930-20120220225642

Exactly, and antagonist is described as an opponent, competitor, enemy, or rival. Luke happens to fit all these. He competes with Percy, is a rival for Annabeth's affection, enemy to the Olympians, and an opponent to pretty much everyone that doesn't join his side