I have played this game until one glitched boss (I can't remember their name, but I believe they were orange and their level used flowers.) in which their key was not collectible due to a flowerpetal's collision being left behind & catching the key.
I adored this game as it was very short, but kept you engaged with the characters and their differing battles! I had trouble initially since I didn't know attacking the rings was necessary until I accidentally hit one. I loved how all the bosses had different ways of fighting you, but none seemed to hurt you aside from delaying your advances (which seems fitting for the story!) The art for the angels was simple but great to tell who has what key and hints at their personality. I just wish I could've not been locked out of the ending by a glitch.