Reach the indicated rank to unlock the corresponding skin:

Gibson (Marine): Rank 2
Claw (Predator): Rank 3
Warrior Dome (Alien): Rank 4
Connor (Marine): Rank 5
Stalker (Predator): Rank 7
Ridged (Alien): Rank 10
Moss (Marine): Rank 13
Hunter (Predator): Rank 16
Praetorian (Alien): Rank 19
Johnson (Marine): Rank 22
Wolf (Predator): Rank 25
Rookie (Marine): Rank 28
Spartan (Predator): Rank 31
Nethead (Alien): Rank 34
Gibson (Marine): Rank 37
Lord (Predator): Rank 40
Van Zandt (Marine): Rank 43