Nico di Angelo
Though Nico was inside of a bronze jar (after getting captured) during most of The Mark of Athena, he does get rescued by Piper, Jason and Percy. Nico also explains to the others where the doors of death are and how to close them. When Percy and Annabeth are holding on for dear life at the edge of a chasm that lead to Tartarus, Nico desperately tries and fails to save them before they fall into Tartarus, promising he would lead the others to the other side of the doors of death.