Cool game! I really liked how each difficulty brought a new mechanic, making the game more interesting. I would actually see this more like a rhythm game, since you always want to press e or space when you have enough stamina, so you have to get into a certain rhythm. I love the undertale setting, although napstablook was a bit annoying every time someone overtook me. I think the only gripes I had have already been adressed by everyone else.
The art is really really pretty, I had quite a bit of fun, even when it got hard. Good job! :))