Get the indicated number of points in the indicated Quick Play challenge to unlock the corresponding character skin:

Dodge Whore: 10 points in Fire Fight
Hollister Thug 1: 150 or more points in Quail Hunt
Hollister Thug 2: 100 or more points in Hold The Fort
Port: 100 or more points in Suppress The Outlaws
Red Hand Thug: 40 or more points in Bear Hunt
Rude: 50 or more points in Hollister's Dynamite Run