Well, that was quite the experience.
I thought that it was pretty boring at first, just kinda thinking that it was another Undertale type game (Didn't help that I was already in an iffy mood) But, after a bit of playing, I got into it! Specifically the final fight of the game, which I think is the best one by far!
The art is visually appealing, the fighting mechanic is really good, even if it resembles most other rhythm games (Bars going either downwards or sideways, notes going down them, so on.) The music was fantastic, and the mix of 3D and 2D in some of the fights was really neat!
Only real problems I have with the game is, as said, I thought that the intro was a bit boring, kinda got into the fight a bit too suddenly when I was still fresh, but I adapted, and that the UI doesn't have much going for it, just a gray bar, but the visuals of the fights more than make up for it. (Plus, making it flash or something would probably be distracting, so that kinda makes that point moot)
Overall, a fun little demo, or slice, as the cool kids call it! Anyone else reading it should play it, and hey, you may love it way more than me!
Sorry if this wasn't a very helpful "review" or anything, but thanks for the few minutes of fun! I'd say that the full game will be great at this rate!