Hey, here's some guy from out of nowhere who has secretly been following this post, and not said anything 'cause shyness. Anyways, I'm gonna jot down some things I noticed while playing the most recent version.
The falling physics seem to be a bit wonky, at least to me. Yuyuko's very floaty in nature, which isn't weird because she's a ghost, but it kinda makes it difficult to try to fall under projectiles while I'm flying about. I did discover pushing X (Inhaling) set her falling speed back to normal, though, so I guess that fixes that.
While there's nothing wrong with a single jump height and flight on its own, sometimes the level design doesn't keep this in mind, specifically in Stage 4 with a drop leading to a small spike hall.
It is here I've also noticed a couple other minor things. Firstly, I think the scrolling may need some tinkering. The screen doesn't like to scroll up or down until Yuyuko tries to pass the borders of the screen, making the aforementioned spikes very difficult to avoid. Secondly, and this is minor, after some time while Yuyuko's falling, she seems to stop in mid-air and start falling again. I figure that's just a minor glitch influenced by her falling speed, but I thought it was worth pointing out.
The mechanic of storing bullets, as opposed to Kirby's method of eating and spitting immediately, is interesting. However, as others may have pointed out, you can just store bullets like a pink-haired squirrel and unload them immediately onto the midbosses. Methinks Yuyuko should possibly be able to store fewer bullets, or maybe the midbosses need a little buff. It doesn't do too much for the Stage 4 bosses, though, as their lifebars are big enough to withstand Yuyuko's machine gunning.
Oddly enough, between the Stage 4's bosses...I found the midboss was actually the more difficult of the two. Yukkurisa as a whole moves faster (but not too fast) than Marisa, making it more difficult to dodge. On top of that, I figured out in the Marisa fight I could stay in the opposite corner from her and be completely safe from most of her attacks. I only need to move a little when she fires star bullets in a circular pattern, or when she performs the attack that gives me ammo to spew at her, which, again, I am safe from in mid-air.
All in all, you have a good thing going here, and it looks like you're going to need more art assets for it...might I be able to contribute to your project here? It's been awhile since I made sprites, but I enjoy drawing a bit.