They wouldn't be able to be go into the camp or anything, they probably wouldn't even have anything to do with the campers. there is 'mortals' as their called aka regular humans, who have ability to see through the mist but they don't tend to want anything to do with it. it is modern times as well. though I take it you have some experience in the books some when having talking about the 'Mist' which was a specific idea from the books. in real mythology nothing was hidden, but that would be to chaotic lol.

If you want a character that mainly just has good luck and cunning you could make a son/daughter of Hermes, they don't have any specific power or anything, they can be lucky at times, very cunning, and love to play pranks.. of course this isn't including them being able to steal very easily somehow lol.