This is great! I see you've received a lot of feedback already on the boss battle, so I'll be short with that: it might be a bit too hard for being the first boss, and the stun mechanic isn't really clear (I think I might've taken advantage of it at random). Other than that, the scanline effect works really well when exploring but made the bullet hell sections very hard to follow. I would offer an option to tone it down on the settings, at least during fights.
The story seems intriguing, and I'm really loving the artstyle and general atmosphere! I'm stuck on the end of the boss' second phase so I guess I couldn't see a big part of the demo, but I'm excited to see how this project unfolds :)