Nico has black hair that always looks like he just got out of bed and olive skin. By The Battle of the Labyrinth, his skin has paled to white, and he takes to wearing black clothing with skeletal designs, a Stygian Iron sword and a silver skull ring. 
Nico is grim and solitary but powerful, he doesn't want to accept his belonging to camp half-blood just as his father and his belonging to the olympus.
As son of Hades he's not easily accepted by others demigods.
He's shy but lovely