Marcus

Marcus was a legionaire and an old comrade of Jason Grace, former praetor of the Twelfth Legion. His rank and cohort at Camp Jupiter are unknown, as is his immortal ancestry. He fought in the Second Demigod Civil War, led by Octavian, centurion of the First Cohort, and reluctantly by Praetor Reyna Avila Ramírez-Arellano.

History
Little is known about Marcus's past. It can be inferred that he was trained at the Wolf House by the wolf goddess Lupa. At some point, he made his way south toward Camp Jupiter and was accepted into the legion by the augor.

Presumably, Marcus fought in the Titan War, since Jason Grace considered him to be one of his old comrades. Whether or not, he fought in the Battle of Mount Othrys is unclear.

After the second fall of the Titans, Marcus likely also fought in the Attack on Camp Jupiter, alongside Reyna Avila Ramírez-Arellano and the new recruit Percy Jackson.

The House of Hades


After confronting the pirate Sciron, Jason fell into unconscious mid-air, dreaming about the Romans' gathering in New York before their planned attack on Camp Half-Blood. Jason recognized Marcus among his old comrades, which also included Dakota, Nathan, and Leila.

Marcus was present when Rachel Dare arrived with Grover Underwood, as ambassadors from Camp Half-Blood. Rachel provided Reyna with a message from Annabeth Chase. Upon this, Reyna departed for Greece, reluctantly leaving Octavian in charge. As soon as Reyna was out of earshot, Octavian told Marcus and the others that there were going to be some changes in the legion's plans.

Trivia

 * The name Marcus is derived from Mars. Given this fact, it is possible that Marcus is a son or descendant of Mars. However, this cannot be proven.
 * Marcus is first referenced in The House of Hades as an old comrade of Jason's. He was not named in The Lost Hero, where a similar list appeared. However, the list in The Lost Hero named only Jason's friends, not his comrades.
 * Marcus shares his name with Marcus Antony, another Roman legionaire from the same era as Octavian's namesake.
 * Marcus's name is similar to the name of a camper from Camp Half-Blood: Mark, son of Ares.