Zoey. It was Zoey!
I would have never guessed she was the third mystery character teased for Super Time Force Ultra following Pyro and Saxton Hale from Team Fortress 2, but it makes sense in retrospect. The Left 4 Dead star brings her dual pistols and propane tanks to the action.
Beyond these characters, Ultra adds 50 Helladeck challenge levels that look worthy of that name and the Ultra Force power mode. Read up on those additions here and prepare for August 25.
STFU ON STEAM ADDS TF2, L4D CHARACTERS! [Super Time Force]