Pretty awesome little game! I got stuck after a certain character perished, and wasn't sure where to go, so I didn't make it too far.
Gameplay:
Gameplay is good. There's a nice variety of moving through areas and platforming.
But jumping feels a little annoying at times. I found myself sometimes hitting a ceiling / platform above me and falling to my death. The spikes in most cases are instant death. I think adding "coyote time" would be helpful. I believe that's where the player is given part of a second after leaving a platform to jump. That would help with timing some of the jumps on moving platforms.
Art:
The visual style and animation is the strongest aspect of this game. It has a unique nightmarish look with distinct character. I love how background elements in the world are also animated (like platforms, or blocks).
Music:
The music is good and the amount of it is impressive for a jam game (level themes, item themes, death theme, etc.). The orchestration and mixing of the tracks is also good!
To be critical, I think the music feels...a little generic. I could see it fitting into any other fantasy type game. I think including some stranger harmonies, odd instruments and peculiar rhythms could have fit the visual style more. Take it as an opportunity to be quirky! I'm thinking of music from Tim Burton + Danny Elfman that might have landed closer to the mark.
Overall, great job! I'd love to play more if I can make it further.