Good foundation, would love to see some more content added to the game with more stats, enemy types, more bosses. Whole lot to work with here!
It looks like there might be some slight i-frames already, but I would definitely suggest upping the amount you get when hit by an enemy - you can get tagged by the enemy bosses bullets multiple times in one volley and get shredded instantly. Not a crazy big deal since you don't lose anything when you di - bweh, but if you want to add more content, there might be some interactions that feel unfair if you can just take huge chunks of damage like that in one go.
But all in all good work, especially the art, 81800 very cute!