Awesome art and way of telling an interesting story!
The main menu music was really loud compared to the normal BGM for me on headphones. Also, dying feels really punishing, having to go through everything again. I think you mentioned on stream, that you changed it so that one can only throw the balls horizontally, to make it more challenging, but I think allowing for aiming with the mouse would have made it more fun, or fair (to me).
The art and style of the sidescrolling level are amazing, I really like the parallax scrolling in the back and the sprites in the foreground. Also, the technique used in the opening, with a static background and animated characters, looks really good.
箱の森だね、素晴らしい