Aeter Red Skin:

-If you beat normal mode, you unlock the Aeter "Fully Red" skin.

Aeter Clown Skin:

-If you beat hard mode, you unlock the Aeter "Clown" skin.

2007 Skin Cheat Code:

If you click the keys "STR" on your keyboard, you unlock the Aeter "2007" skin.

Easy Mode Flawless

-If you don't lose any rounds in normal mode, you get the Aeter "Britney Spears" skin.

Secret Gun Skin:

-If you win level 4 on hard mode only using the gun, you unlock a new gun skin.

Hard Mode Flawless/Sarah Skin:

-If you don't lose any rounds in hard mode, you unlock the "Sarah" skin.