He had another sibling and they were trapped, that was all Nico was told when he was sent on a quest by his father to find his sibling. He went in blind willingy, he was determined to find them for some reason.

The black haired boy immediately went and told Hazel, telling her that he'd find their sibling, bring them back from wherever they were trapped. He promised her.

It wasn't a difficult promise to make, he wanted to find them anyway, he needed to find them.

The Italitan first searched the underworld, which, gave him no luck. Next, he searched both Camp Half-Blood and Camp Jupiter just to be sure there weren't any demigods who were in any kind of trouble that could be his sibling.

And now, he didn't know where to look? He had to find them, but he wasn't being given any better clues.