This game really nails the characters. The writing is great, as is the visuals. If the ground tileset was better, this would have been 5/5 in visuals. Also consider adding camera limits.
The main problem I have is with the level. It needs to be bigger, with more incentives to go to new places. I'm also pretty sure some section requires invincibility to pass through (like going down to the bottom level again), which is fine, but it would be nice if this was made more obvious so the player isn't baited into trying. It doesn't help that Youmu gets stuck in the bottom section, which incentivizes the player to just not go there at all, and just farm respawning spirits at the top level.
Sound effects could have helped the player know when they've gotten to the next level in an upgrade.
The foundation is solid enough that with just a little more effort, it would have been a much better game.