Nexon just announced that the Thief class, along with her Rogue and Necromancer subclasses, will be coming to Dungeon Fighter Online this summer as part of the Act X: Rebirth update. The patch will also mark the removal of Fatigue Points from the game. They'll be replaced with Blitz Points, which will let you play for as long as you want and reward you with bonus exp.
More Act X: Rebirth details will be posted on the update's teaser site on June 1 and June 15. While you wait, you can watch the anime Thief introduction video below.