It's a very short demo, but i'm already a fan of this. I love the art, animation and everything you put into it. As you requested, here are some issues with the game:
Framerate issues, not in the animation, but the game just doesn't run very smoothly.
In the part where the screen doesn't scroll, the player or enemies can appear off-screen making them unable to see. So, just a little bug.
The control scheme, although I understand it's personal preference, didn't feel great. Maybe just use the up arrow to jump?
Lastly, the English was flawed in some places. I understand it if English is not your first language, that's okay. Maybe get someone to check and correct some of the sentences? Heck, I'm already a big fan of this, I'd be honored to do the job if you're interested.
4/5, a great demo. I would love to see more!