I really enjoyed this! It threads the needle in being just the right amount of difficult, it took me a fair amount of tries between each checkpoint, and also on the boss.
After getting used to the movement, fall speed and just how far the air-dash takes you, handling the character feels great and it felt like each challenge was set up the right way to show you what the character can do, which is a great thing for a first level: it was less like "how am I supposed to do that", and more "that's what I'm supposed to do, but man it's gonna be a bit tough". I saw some little parts where you can optimise and make skips too, which is going to be cool for the speedrunning crowd.
My thought while playing was also "Hmm, this seems a bit harsh, 5 hitpoints to get through all that?", and maybe the checkpoints should be closer, but actually that is solved with the pause screen option to refill health every room. The placement of the checkpoints is actually just right considering that, there's a natural rise and fall of tension/relief when playing. My only consideration on that would be "should the first level overall be this hard?". With a standalone trial it's fine as it occupies a decent amount of time, but it could be offputting for a beginner when they start with this and can only imagine that it's going to become more difficult as they progress.
My only other piece of feedback would be to potentially add some more elements to the backgrounds, not necessarily to add visual noise for the sake of it, because the aesthetic is already good & because colourful backgrounds would distract from the on-screen hazards. But it could be a way to potentially weave in some interesting background details in the less perilous areas, or empty-ish corner of some rooms, maybe even serve as a way to loosely feed in some narrative/world-building.
All in all, I'm really excited to see how this develops!