Mark

Mark belongs to the Ares cabin, and is a son of Ares.

The Lightning Thief
Mark is probably one of the Ares cabin that gangs up on Percy Jackson and attacks him. When they push Percy into the lake, Percy feels his strength returning and attacks them all, beating every one of them.

Mark might have been one of the Ares cabin that made Percy's ugly shroud when he was on his quest.

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The Demigod Files
When Clarisse mentions that her brothers are messing with her, Percy instantly thinks of Mark and Sherman, which probably means they are the most troublesome.

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The Last Olympian
Mark was one of the Ares cabin that was convinced by disguised Silena Beauregard to join the rest of the Campers in New York City in the battle against Kronos and his monsters.
 * 1) Mark has been trained in hand-to-hand combat at Camp Half-Blood.
 * 2) He is very strong and swift.
 * 3) He may have some small degree of powers that Ares himself possesses.
 * 4) 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.