Mumei always has the coolest games. I really liked how the backgrounds and music changes. It gives off the sense of a journey happening. I also liked the events (trees falling/breaking) as it kept me wondering what would happen next and looking forward to exploring.
Since the game is in progress, I'll try to do some bug reports/feedback to hopefully help a little bit.
Main design feedback is that After getting the double jump, I immediately started walking left until I reached the place where I initially dropped down into the underground city. There was a platform that looked JUST out of reach from the double jump and I tried a lot to get there.
It wasn't until I went back and jumped around at random that I discovered that you were supposed to jump up from the roof of the structure to find more platforms. Something like an arrow prop or a strategically placed wall/platform can help people find the intended path a little easier.
Double jumping also appears to be somewhat consistent on sloped platforms which made some of the underground city sections a bit difficult.
There are also some issues you maybe already know about:
- In the room where you get a double jump, the yellow trigger hitbox is visible
- In the room where you get the double jump, if you jump before picking up the letter you can fly into the air.
- There are some scenes which have the temporary Mumei dialog sprite still (the scene where the stalactite falls and makes a hole).
I like the story that you have! It really makes me wonder when the game takes place and how long it has been since Mumei was in Hololive.