Just got around to playing your demo!
The controls feel tight and the gameplay is solid. I really like how you had something new to introduce in each of the demo levels, so that kept the demo feeling fresh the whole way through, and each of the levels was able to stand out from all the others. The visuals are quite clean and pleasing to look at as well! Some of the levels being labeled as "secret" do make me curious if you have plans to include future levels with branching paths in the full game, as everything here is linear, and I don't really see room where you could hide exits to secret levels. Overall, this is a very promising demo that feels very refined compared to your games from the past!
Wall-jumping and attacking large enemies are the only things I really found issue with. There doesn't seem to be any way to disengage from a wall if you touch one while airborne, so you either have to wall-jump or wait until you slide down the wall. This isn't a significant issue in any of the current level designs, but it was something I noticed that felt off. The loss of control when attacking the large enemy or the boss make sense to me since it's intended to push you away, but their hitboxes are so large that I got stuck inside them and ended up taking hits fairly often. I ended up doing a ton of damage to the boss in a short time because I would get stuck bouncing back and forth inside the hitbox, which was pretty funny, but it definitely didn't feel intended.
Oh yeah, I had one more comment about the flying enemy that shoots fireballs on the Skyway level; there's no visual warning before it fires another projectile, and although it's not really an issue since its attack is on a consistent timer, I feel like adding an additional sprite frame right before it attacks would be an appreciated warning and detail.