Who is Nico di Angelo. from Heroes of Olympus, the, Book Two: The Son of Neptune and what is their importance?

Nico di Angelo is Hazel's half-brother..... a son of Pluto. Nico was in the underworld attempting to save another sister when he discivered Hazel and brought her back to life. Later in the novel, he is captured by Gaea. Nico knows Percy, but pretends he doesn't when Percy arrives at Camp Jupiter.