[Nico blinks.] Oh, wow, you must come from somewhere that hasn't had a monarchy in ages.

A prince is the son of the current monarch. Technically I am one since my father rules the Underworld, but it's not like he's ever going to give up the job. And even if he did, he has older children.

[Children that are divine in nature, though Nico's only met one and she wasn't pleasant]