You can unlock additional skins by reaching the following levels:

• Alien / Nethead – Achieve Rank 34
• Alien / Praetorian – Achieve Rank 19
• Alien / Ridged – Achieve Rank 10
• Alien / Warrior Dome – Achieve Rank 04
• Marine / Connor – Achieve Rank 05
• Marine / Gibson – Achieve Rank 02
• Marine / Johnson – Achieve Rank 22
• Marine / Moss – Achieve Rank 13
• Marine / Rookie – Achieve Rank 28
• Predator / Claw – Achieve Rank 03
• Predator / Hunter – Achieve Rank 16
• Predator / Spartan – Achieve Rank 31
• Predator / Stalker – Achieve Rank 07
• Predator / Wolf – Achieve Rank 25