I definitely appreciate how much content is in this demo. This is certainly shaping up to be a proper game. I have a couple of pieces of advice/feedback that I hope are constructive:
1. For the movement mechanics I think you should tweak the acceleration and deceleration values a bit. It feels too slow to start moving and too slow to stop moving and the result is that it's hard to place the characters where you want to whether in air or on the ground. Higher acceleration and deceleration constants would probably be sufficient for tweaking this (while keeping the same max speed), but if you want to get fancy, it might be worth it to add a different, stronger value for "turning deceleration" (i.e. momentum is going right, but you're pressing left) and potentially a two-phase acceleration where up until, say, 70% of max speed you use a very high acceleration, but then you slow down the acceleration so it takes a bit more to get to max speed (this is something that's pretty common in old-school platformers).
2. Personally, I would very much advise against the use of physics-engine gimmicks in a traditional mechanics-based platformer. With this I am referring primarily to the ball pit, but also the blood pool in the boss fight is somewhat this (although, impressively, you managed the mechanics well enough with the boss that the water physics weren't really an obstacle). With physics engines come chaos (i.e. a small change in input/starting parameters = a large change in outcome. Butterfly effect) and with chaos destroys design and intentionality. These parts feel more like "because I can" moments than "because I should" moments.
Anyway, great to see a game really shaping up. The characters are cute. Dialogue's a bit cliche, but it fits.