Rick Riordan said this quote
“ 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. 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. ”