Mark

Mark is a Greek demigod son of Ares.

The Stolen Chariot
When Clarisse mentions that her brothers are messing with her, Percy instantly thinks of Mark and Sherman, which possibly means they are the most troublesome of her siblings.

The Last Olympian
Mark may have been one of the Ares campers that was convinced by a disguised Silena Beauregard to join the rest of the Campers in New York City in the battle against Kronos and his monsters. Only later did they realized that it was Silena and not Clarisse.

Demigod Abilities

 * Mark has been trained in hand-to-hand combat at Camp Half-Blood.
 * He is very strong and swift.
 * He may have some small degree of powers that Ares himself possesses like using red fire or changing his weapon form like Ares did in the The Lighting Thief.
 * Being a child of Ares would make him an expert in all weapon usage and may even have some degree of magical control over them.

Trivia

 * The name Mark is derived from Mars, the Roman form of Mark's father, Ares.