Really impressive! Feedback, amount of upgrades, juiciness. I'm amazed you managed to cram all of this into the jam time and have it look and feel this polished. Excellent work.
Things that didn't quite click with me (and honestly, these are all quite nitpicky except the first one):
Hovering over your character with the mouse pauses the game and shows your stats:
It's a neat idea, but I found myself activating it a lot unintentionally, interrupting the flow of the game.
I actually wasn't sure what was happening at first when the little box popped up and thought it was the stats of the enemy. I'd just keep that information available on the pause menu and emphasize that you can press escape to pause and view stats. Getting any kind of unintentional pause in the action (that isn't to pick an upgrade), when the action is this good should be avoided.
Ghost Walk:
As Snek mentioned, the ghost walk did feel a bit awkward and jarring. I 200% get the effect you were going for however - just didnt feel it worked quite well for this style of game.
Damage values that appear from enemies:
I'd make them rise up a bit besides just moving in the direction the enemy is knocked back. They were kind of hard to see. I'd multiply the values DISPLAYED (not actually applied) by a factor of 100 to 1,000 to make the numbers look bigger and more impressive. Good ole psychology.
The EXP pickups:
I'd use a more contrasting color. I like how the shapes grew to show their value, but (especially in the beginning when they are super tiny), they were kind of hard to see amongst the enemies. I can tell you picked a pallet and stuck with it, which is great for a cohesive style, but that can also be one of the downsides.
Played it a few times to build on my critique and gather my thoughts. I got to about the 13 minute mark on my last run. Again, really great VS clone and I'm super impressed. :)