Tobias: Tobias isn't big on nicknames so you just call him either Tobias or Four. Very simple, but he loves it anyways.
Eric: You call Eric, Captain. He is a powerful and natural-born leader. He loves it when you call him that.
Caleb: Caleb is constantly correcting people, so you call him Smartass. He wasn't fond of the name at first, but has learned you do it out of love.
Peter: You love calling him Sweetheart. Peter hardly looks "sweet" at the surface, but behind closed doors, he's the sweetest person alive.
Uriah: You call Uriah Cat, which he despises. You call him that because he sounds like a strangled cat when he goes on the zip line. Because of this, he doesn't like to zip line anymore. He gets pretty embarrassed which you find very, very cute.