Goldmember released a Left 4 Dead 2 character replacement mod featuring Krystal. Once installed, Krystal (in her Assault uniform) replaces the character Rochelle.
For those who don't know, Left 4 Dead 2 is a multiplayer zombie adventure-survival game. So, you know, gore and stuff. Fair warning.
Somewhat related: there's also a Wolf mod as well.
Thanks to Dogman15 for the info.