It's a fun game, especially for being developed in 3 days! I played way too much of it when I was bored (as you can tell), and I enjoyed it quite a bit.
Because I played it for so long, naturally I have some feedback of things I noticed. There are obvious improvements like expanding the gameplay with more enemy types, more room layouts, etc, however two more niche things that could be improved are:
Movement speed is capped, somewhat like bullet damage (the practical cap being reaching enough damage to one-shot every enemy). However, you can still stack more bullet damage after you hit this point, and use this to your advantage later. Movement speed is capped at a certain value, and increasing it further does not actually increment the value past that. (I can tell because I got a +5.63 move speed increase once, and my speed did not noticeably change. However, I picked up a -2 move speed decrease and Youmu slowed dramatically.) This was a bit confusing at first, but this could be solved by displaying both damage and movement speed values on screen, along with HP and SP.
There's a glitch where sometimes your SP would be set to your HP value after entering a new level. I don't know what causes it, but the SP value resets back to its cap once it changes (all times this occurred to me, my current HP value was over my SP cap.) Just thought y'all should know in case you weren't aware.
(Also minor issue, there's no way to exit the game as far as I can tell.)
Again, great game, and I hope these suggestions help a bit! I look forward to future updates!