Not a bad little game at all! You achieved one of the key virtues of a game jam; being feature-complete and properly scoped as to allow polish. I appreciated the longer run-up time you gave to let me get used to handling the harpoon and there was a genuine moment of tension when I kept missing my first devil-horned enemy!
There's a lot of little stuff you could do to improve this going forward; stuff like tweaking the pick-up mechanics, creating more varied enemy types, to even more complicated stuff like giving the player new movement options. This game is a really solid base you could go a lot of directions from, and I hope you keep developing it.