Wow, what a wonderful game! Definitely reminded me of Nameless.
The art was very cute, especially the ending screen.
The team wrote Mumei's dialogue perfectly, all of her commentary on the letters felt very on brand, and did a fantastic job using them to tie into the theme - I was concerned about how it was incorporated until the second half of the game.
Speaking of, I love how the music starts building up as you begin to ascend back home, and I like the little touch with Mumei's dialogue on pausing and unpausing the game.
If I had to log a complaint, the physics were kinda janky sometimes on the inclined platforms - the main issue was that jumping while sliding off of them ate the double jump, instead of the regular jump, so needing to time precise jumps didn't feel all to great sometimes.
Thanks about it, though. Great work, and fun!