Ares is the secondary antagonist of Rick Riordan's 2005 fantasy novel The Lightning Thief. He is based on the god of war on the same name from Greek mythology.
Though he appeared at first to be an ally to protagonist Percy Jackson, it was eventually revealed that Ares had been using the heroes in order to start a war among the gods. It was further revealed that the war god had been a pawn of the main antagonist of the series, Kronos. At the climax of the novel, Percy confronted Ares in combat and against all odds defeated the god of war in a duel. In losing the battle, Ares declared his hatred for Percy Jackson, then vanished.
In the final chapter of the novel, the true main antagonist Luke Castellan revealed that after he had stolen Zeus's master bolt and Hades's Helm of Darkness, he had been immediately captured by Ares. The war god threatened to hand the weapons over the gods and have Luke face the consequences. However, Kronos spoke through Luke and planted the idea of a war among the gods in Ares's mind. Hooked, Ares decided to hold onto the weapons and help Luke frame Percy Jackson for the theft. Had this plan succeeded, Ares would have unknowingly played a key role in the revival of the Olympians' oldest enemy, as well as the destruction of his own people and Western civilization.
In the following novels, Ares became far less antagonistic, despite his continued hatred for Percy. Therefore, he would be considered a oneshot villain, for is only an antagonist in The Lightning Thief.