Okay now this game was fun as flipping hell, I have to say. Maybe it was just my bias to kaizo-ish games plus playing on expert mode, but wow, I had a blast with this!
I actually don't have a lot to comment, everything here feels pretty good already, from gameplay, art, and etc. Altought music takes all the spotlight for me honestly, it's catchy, melodic, upbeat and just exactly the kind of music I wanna hear on vibeo-games, music that pumps me up while I'm playing!! (In fact, the fact I was enoying the music so much all of the time might as well be another reason I don't have much critiques, because thanks to it I wasn't really paying a lot of attention to the game itself at all xD)
Boss patterns also?? They're top top-tier, those were super fun fights, nothing to comment about, just wanted to mention it. (Altought I do admit they felt a little damage spongy in the end, since they doesn't ramp up their tricks as the fight goes on, so there's no reason for their hp bar to be so big, wich is a bummer, so forget what I said, they're only top-tier, but hey, that's already pretty good am I right)
I guess it's not perfect tho, as other people probably already mentioned, this game definitelly needed a map, or more landmarks, because dear god this game is huuuuugee (I usually vote for landmarks instead of maps, because they make the world itself feel more alive, altought ideally, you'd want to have both). Honestly I'd say that this game feels huge on the wrong way, since it's not huge as in "oh wow, how many cool places I'm visiting!", It's huge as in "oh wow, how many rooms I'm walking throught", wich, granted, the rooms are all super fun to traverse, but still, it gets old eventually. Not to mention you actually made us backtrack!!! Wich is a HUGE plus on a metroidvania, of course, but witouth a map or memorable spots to guide you, I'm afraid people will more often than not just give up exploring (I myself only knew I was supposed to backtrack after getting the slow-fall because I remenbered the big pit room with the 3 jellyfishes, and believe it or not, that was a notable landmark! You just needed a few more rooms that felt actually different like that one, visuals help, but they aren't the only way to do so.)
Also minor nitpicks: I wish the game saved automatically the itens I picked up instead of having to reach a save point, otherwise if you die on the path you'll need to do everything all over again, wich can be a bit of a chore considering how far all of those itens are from any save point. (Granted, on my gameplay that resulted on a hylarious Moori re-fight because I killed myself trying to rocket jump after getting the upgrade, wich I always find very funny, but you certainly wouldn't want that to happen with someone with a shorter fuse lol). Also, something more personal, but if you're not using W for anything, pls let me jump with it, I don't like Spacebar.
Lastly, I actually didn't finish the game yet, I got the lantern, but stopped after that. As I said, this game is huge, so I kinda needed a break after playing it for an entire hour lol.
Here's the gameplay vid for that hour tho, if you have the patience to watch it: