Absolutely outstanding art. I love the extremely restricted palette. Concept and mechanics are also amazing - I mean seriously, making a Kirby bullet-hell is incredibly clever!
I do have two fairly large issues though. For starters, the player's hitbox is too large. Other bullet-hell games have tiny hitboxes to make it easier to dodge - Undertale's heart is tiny compared to the battle scene and most attacks, for instance. Some bullet-hells have hitboxes that are only the very center of a character, to get both a detailed character and easier dodging.
Related to this is that the movement speed after inhaling a bullet is too slow as well. This is fine, but combined with a large hitbox size (which I think get larger after inhaling?) makes some attacks near-impossible to dodge. Moving while having a bullet ready should be a risk, but it's too much of a risk right now.
Overall, I still think the game's fantastic, and absolutely deserves to be expanded on. It's just a little too hard for most people at the moment.