Greetings! The demo was really fun to play! The game seems to be a direct upgrade from UdongeinX1 across the board, and I absolutely love the choice of having multiple characters be playable.
The main feedback I'd like to give is that the level and boss design does not seem to favor Seiran very much. Hitting the tanukis that are next to the little holes with the tsuchinokos is very difficult, and the boss' attack patterns also leave very little openings to be able to get in and deal damage. I also think the temporary invincibility it gets heavily restricts the reward the player gets for trying to get as many hits in as possible. But I really like the effect that happens when the room gets dark, and the attacks are very cool.
I'm excited to see how the rest of the game turns out. UdongeinX has been a pretty huge inspiration in my current project, so I wish you very good luck with the development of this one.