Successfully complete story mode under the normal difficulty setting. You will unlock a new skin or level for each level completed.

Biker 2 skin: Chapter 1
Bar level: Chapter 2
Biker Boss skin: Chapter 3
Triad 2 skin: Chapter 4
Jack 2 skin: Chapter 5
Triad Boss skin: Chapter 6
Militia 2 skin: Chapter 7
Militia Boss skin: Chapter 8
Cabal 1 skin: Chapter 9
Cabal 2 skin: Chapter 10
Mansion level: Chapter 11
Whisper skin: Chapter 12