Fun, but...
Here we go.
First of all, I have to say that this feels like the end of a hack and slash platformer, and without that portion, it feels incomplete.
Bugs - Simply too many.
-Vastly uneven drops: It's nearly impossible to upgrade the sword past lvl 3 because of the rarity of the last 2 gems.
-Gem drop/pickup mistake: The last 2 gems are switched.
-Unbalanced stats: Example - If you put a lot of points into Strength, you build up damage quickly, yet when you pour points into Agility, he still rarely misses you.
-There are others, as other people have pointed out, these are just the most aggravating
Other complaints
-Speedy thief: he goes a little too fast to catch up with him if you get hit.
-Lack of HP recovery: The only way to get HPs is to level, and the only way to recover them is to die, which you can only do once.
-Constant interruption: I was hard pressed to find much time to chase down the thief for gems with the boss and the flame enemies constantly bearing down on me.
-Too few different animated poses: His stopped pose is the same as his jumping/falling pose, and it doesn't look right. It looks like it should only be for sliding (to a stop while running, or while changing directions). Though, the few (3-4) different sword strikes that you have are alright.
-Gravity seems odd: maybe make the gravity a little stronger, and incorporate a double jump instead of the low-g jump you have going.
I hope you don't hate me now, it was a fun game :) keep going, keep improving.