Very good looking game and nice atmosphere.
The level design was pretty good, although I got a bit lost sometimes because the checkpoints were a bit too far and disorienting from the place I fell down.
I think the movement could need some work, the character falls very fast once he reaches the apex so it feels very stiff and "springy", for example the way to cross a large gap is actually to jump the highest possible to compensate for the fast falling movement.
I don't think the movement is poorly designed or "feels bad" per se, however it does feel somewhat at odds with what you were going for. A slow moving character who's going deep into themselves should probably have overall slower vertical movement, for example. Though this specific solution is subjective, I think that keeping in mind that in a platformer, the way a character moves actually tells you a lot about its personality and can evoke specific feelings will help you a lot when making this kind of atmostpheric / narrative platformers. The slow horizontal speed, for example, was a nice touch.
The sound design and atmosphere works, although I would say the soundsteps repeating always in the same sequence got a bit repetitive after a while. Maybe randomize them a bit ?
Anyway, great game, I just wanted to give some constructive feedback, hope it's useful !