Great aesthetics. I love the menu's look and feel, and the animation in the game itself is great. I had to turn the brightness up from the default to see enemies clearly, but that could've just been how my display is configured (I'm using a TV instead of a traditional monitor).
I liked the "floaty" feeling attached to the main character -- the drawn-out leaps and slides were cool and felt good. From a design standpoint, it seems like sliding is generally safer than jumping -- it gets rid of enemies and avoids bullets. Maybe I just didn't get to a point where there are enemies that aren't affected by sliding, but it might be something to think about from a balance perspective.
One other small note - the character running sound effect makes an abrupt jump from left to right speaker once you cross the middle of the screen. I found it was a little distracting, especially while using headphones. It might be worth going back to see if you can make the audio pan more smoothly from L to R.