Quote from : The House of Hades Book

Images flashed through his mind. He saw Nico and his sister on a snowy cliff in Maine, Percy Jackson protecting them from a manticore. Percy's sword gleamed in the dark. He'd been the first demigod Nico had ever seen in action. Later, at Camp Half-Blood, Percy took Nico by the arm, promising to keep his sister Bianca safe. Nico believed him. Nico looked into his sea-green eyes and thought, He was Nico's favorite game, Mythomagic, brought to life. Jason saw the moment when Percy returned and told Nico that Bianca was dead. Nico had screamed and called him a liar. He'd felt betrayed, but still... when the skeleton warriors attacked, he couldn't let them harm Percy. Nico had called on the earth to swallow them up, and then he'd run away- terrified of his own powers, and his own emotions.