Desmond was supposed to be a part of Lucius's group pushing deeper into the area where the undead tended to originate every night. However, at the last-minute plans were changed up and he was asked to assist in the defense of the camp.
Desmond held no complaints about the request because it did mean he could keep a better eye on his sister who was not capable of fighting in a place like this. She had no Compatibilities at all and her mark was rather useless in an area like this.
Jade possessed the Mark of Water Control. If she were near a body of water she could manipulate it to her heart's content. At their academy back home, she was a rather terrifying opponent as it was on a small island and they were surrounded by infinite amounts of ammo for her to use whenever and however she desired.