Nico di Angelo
Nico is supposedly eleven years old in this book. He is first seen talking with an unknown ghost who is trying to tell Nico how to trade his sister's soul for another soul, when Percy is sent a collect Iris Message sent by Bianca Di Angelo, Nico's sister. It is revealed in this book that Nico can summon the dead. Nico is then seen when Percy, Annabeth, Grover, and Tyson visit Triple G Ranch, where Percy then saves Nico from being sent to the Titans. Nico, still angry at Percy, finally understands that it was Bianca's choice to go into the Talos robot, and that he shouldn't be mad at Percy. The others leave Nico at the Ranch, where he supposedly meets King Minos (in ghost form) again, who convinces him to go back into the Labyrinth to supposedly save Percy and the others.