It appears Nico is harder to explain more than the others... So, I'll mention the main things, Nico is extreamly loyal, brave and bold, and very caring to those he is close to, and has a short temper. Yeah, he's a bit dark and stuff. Actually, you can read it for yourself becasue I am bad at this X'D http://riordan.wikia.com/wiki/Nico_di_Angelo