I like the concept! Movement + arrow keys to attack is neat, and managing upgrades while under attack is always fun.
I think binding keys to level up stats is useful, but not very intuitive for new players. I found myself lifting my hand off the keyboard to use the mouse to click the buttons instead, which meant I couldn't attack. And then because I didn't want to memorize all the button inputs, I just spammed E a bunch.
Overall, the sprites are pretty clear in their intent, but there is a missing cohesiveness to the art overall. I think restricting yourself to a palette and sticking to it would help! That, and maybe consider drawing all the characters on the same sheet - it seems like the characters have very different proportions.
Well done! Great game!