Great game! I also like the art!
The concept of joining with other characters is quite common in the jam but your game is the one of the most fun of those games because of the level design and the fact that we can get new abilities along the way.
One suggestion I have is to add something to indicate how much the jump is charged, something like a progress bar that shows up when you charge jumps and tells you how much charged the jump is.
Overall an amazing entry to the jam! Great job!