This game is pretty fun and I feel like I might get a bit nitpicky but here we go. I like the idea of transforming into a stronger version of yourself. That's a cool gimmick. The spritework and music are also very clean. I especially like the intro to the game, the player sprite, and the Fauna sprite. The Fauna boss fight was also pretty challenging and I liked playing through it. The ultimate was also pretty funny. Having her summon pebbles from the sky is awesome.
I actually think that the charged attack is kind of useless since its so short ranged and you have to basically run up to the enemy to use it. That means you just get hit by whatever Fauna wants to throw at you. Also this might be a hot take, but I didn't really like having the charge attack be mapped to the same button as the regular one. I personally just like being allowed to hold down the fire button for rapidfire instead of mashing it.
Being allowed to fire to the left was also a bit annoying since most of the enemies come from the right so it made strafing harder. The enemies that did come from the left also caught me off guard a lot of the time since that's where I was. I'd maybe make it so you could only fire to the right and all the enemies spawned in on the right side of the screen.
I also thought the tornado hitboxes were too big. Since the Biboo sprite is so big, at least compared to how big your bullets are, it might be worth making the hurtbox a bit smaller as well? I kept getting hit by the spread attacks by Fauna when I feel like I dodged them. Trails would also be nice for when the charging enemies are coming at you or for enemy bullets so that its easier to tell where they're going at a glance when there is so much happening, especially in the later screens.
Speaking of Fauna, I noticed she had two different spread attacks: the one that aims at you and the one that just goes in a circle. I actually like the circle one a lot more since its more predictable and easier to dodge, whereas the one that aims at you got me into some undodgable positions. Maybe consider swapping them or getting rid of the aiming one in a future build and replacing it with a different one? idk. Bullet patterns are hard.
Anyway, this is still a fun and cute game. I really liked playing it and I think the concept could go crazy if you ever decide to take this further.