Mars

"I’m the god of Rome, child. I am the god of military might used for a righteous cause. I protect the legions. I am happy to crush my enemies underfoot, but I don’t fight without reason. I don’t want war without end."

- Mars

Mars is Ares' Roman counterpart. As Mars, he becomes more disciplined as well as even more militaristic and warlike which is shown by his focus on martial victory rather than sheer conflict. In his Roman form, Mars wears a uniform of the Canadian Special Forces. He has children and may also have descendants at Camp Jupiter near San Francisco. The Greeks envisioned Ares as a brutal and fearsome being. The Romans considered Mars Ultor, meaning "Mars the Avenger", to be the greatest of the gods after Jupiter. The planet Mars was named after him.

History
Mars is the god of war, and one of the most prominent and worshiped gods. He is the son of Jupiter and Juno. According to some sources, Mars is the father of Romulus and Remus by the Vestal Rhea Silvia. Because he was the father of these legendary founders of Rome, and thus of the Roman people, the Romans styled themselves 'sons of Mars'.

The fiercest and most ravenous creatures were consecrated to Mars: the horse, for his vigor; the wolf, for his rapacity and quickness of sight; the dog, for his vigilance; and he delighted in the pyre, the cock, and the vulture. He was the reputed enemy of Minerva, the goddess of wisdom and arts, because in time of war they are trampled on, without respect, as well as learning and justice.

Representation
Ancient monuments represent this deity as of unusual stature, armed with a helmet, shield, and spear, sometimes naked, sometimes in a military habit; sometimes with a beard, and sometimes without. He is often described riding in a chariot, drawn by furious horses, completely armed, and extending his spear with one hand, while, with the other, he grasps a sword imbued with blood.

The Son of Neptune
Mars appeared in The Son of Neptune, after the War Games to claim Frank in person, and to give him the quest to go to Alaska and free Thanatos, the god of death. He also includes Percy in the quest, saying Percy needed to learn respect for him. He appears to be different from Ares, because he only likes war if it is for a reason, and disliked endless carnage, something Ares appears to thrive on. He also appears to be more of a strategist, as seen when he gives Frank "The Art of War", a famous book on war strategies.

Trivia

 * Martes and Mardi, meaning Tuesday in Spanish and French respectively, were named after the god of war.
 * In the Son of Neptune, Mars couldn't recall his fight with Percy as Ares, though he odds are he pretended that he could not remember.