Nico di Angelo is a 15-year-old [1] Italian-born Greek demigod, the son of Hades and Maria di Angelo. He's also the younger brother of the late Bianca di Angelo and the paternal half-brother of Hazel Levesque. He's known in the Underworld as the "Ghost King" after taking the title from Minos in The Battle of the Labyrinth. He is currently in a relationship with Will Solace. 

Annabeth mentions that Nico is searching for Percy along with Tyson, Grover, and the Hunters of Artemis, but otherwise, he doesn't appear in this book. It's to be assumed that he doesn't stay at Camp Half-Blood much, as Annabeth said he "sometimes" shows up, which could be due to his double duty at Camp Jupiter as Ambassador of Pluto. She mentions him as one of the only children of the big three along with Percy Jacksonand Thalia Grace. Unknowingly to her, Jason Graceis also a child of the big three, being the son of Jupiter and Hazel, the daughter of Pluto.