I really like the art style and music! The bullet hell gameplay is also really fun!
I did notice the start button is off the side when I don't play fullscreen.
Another thing I noticed is that if you don't kill the tutorial enemy, you can get trapped after going into the next room.
I think having some sort of way to see how much health a enemy has would be really nice. I also thing some sort of score/high score system would be fun and increase replayability!
Overall I really like the game and think it has a ton of potential :)