|“||Marcus, bring Scipio from the stables.||”|
Marcus is a legionnaire and an old comrade of Jason Grace. His rank and cohort at Camp Jupiter are unknown, as is his immortal divine 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.
Little is known about Marcus' 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 augur.
After confronting the pirate Sciron, Jason fell into unconscious mid-air, dreaming about the Romans' gathering in New York City 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 Elizabeth Dare arrived with Grover Underwood, as ambassadors from Camp Half-Blood. Rachel provided Reyna with a message from Annabeth Chase. Upon this, Reyna ordered Marcus to bring her pegasus Scipio from the stables for her departure to 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.
- 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 legionnaire from the same era as Octavian's namesake.