Every time you beat a different mission on Hell Mode, you'll unlock bonus armor. Metal Wolf's character model is essentially the same, but with different colors and patterns.

Skin	How to unlock
Camp Armor	Beat a different mission on Hell Mode.
China Armor	Beat a different mission on Hell Mode.
d Stripes Armor	Beat a different mission on Hell Mode.
Desert Armor	Beat a different mission on Hell Mode.
Doberman Armor	Beat a different mission on Hell Mode.
Dolphin Armor	Beat a different mission on Hell Mode.
Help Armor	Beat a different mission on Hell Mode.
Pinstripe Armor	Beat a different mission on Hell Mode.
Redeye Armor	Beat a different mission on Hell Mode.
Richards! Armor	Beat a different mission on Hell Mode.
Stars and Stripes Armor	Beat a different mission on Hell Mode.
White Sands Armor	Beat a different mission on Hell Mode.
Zebra Armor	Beat a different mission on Hell Mode.