Has unlimited sling. Can use any sword. She can double jump and jump from the sides of the screen. Her most useful skill is the ability to open chests without a key. Also if she runs into creatures they drop items.
Thief can also backstab monsters.
No-one seems to be able to backstab bosses at the moment. To backstab monster must not be facing you and close to you. Quarter-circle with and added twist to it. Also can double-jump and leap off the side of the screen. After the first brief part of the stage, you get to type in your name.