Great game and I loved the art style!
I really enjoyed it, but also have some advice on how it could be improved:
I wish you could pass through platforms if you're already holding S while landing, instead of having to time the button press, and while I like the variable jump height based on how long you hold W, it can make the double jump a bit inconsistent. If there were 2 set jump heights - one for tap and one for hold - it would be easier to know how high you're going to jump each time.
Finally, this is purely visual, but it would be really neat if you defeated enemies you run through while your powerup is active, the same way as shooting them.