Nathan

Nathan is a legionaire and an old comrade of Jason Grace. 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 Nathan'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 augur.

Presumably, Nathan 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, Nathan 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 Nathan among his old comrades, which also included Dakota, Marcus, and Leila.

Nathan 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 Nathan and the others that there were going to be some changes in the legion's plans.

Trivia

 * Nathan shares his name with Nathan Fillion, the actor who portrays Hermes in Percy Jackson: Sea of Monsters. Considering Rick Riordan's attitude towards the films, this is unlikely a subtle nod to Nathan's immortal ancestry's being Mercury.
 * The biblical Nathan was a prophet, which could nod to Nathan's being a son or descendant of Apollo, god of prophecy.