Ravenlok

Outside the Witch's house is a stand with a mouse/rat, this is Decker, talk to Decker to level up your character. You will use "feathers" to do this, you gain feathers (xp) by defeating enemies/bosses. You can pause and see how many feathers you have in the top right corner.
Note: My xbox stopped tracking these achievements, but I kept leveling up anyway and it still registered/unlocked. So don't be too put off if your tracker/progression doesn't seem to be moving!