I enjoyed the demo a lot!! Puzzles felt generally satisfying, the writing is charming, the dynamic between the main characters is very sweet, the art looks good, the game feels good to play, and the music is great. I do have a pile of feedback which I'm including just in case you'd find it helpful. These are almost all nitpicks and it wouldn't surprise me if some of the things I'm about to list are things you already considered and wanted to include in the demo, so please, ignore some or all of this at your leisure.
One thing I note about the game's puzzles in general is that there's a lot of backtracking by nature. This is fine, backtracking is a part of good environmental puzzling, but it can sometimes lead to a lot of redoing old platforming, especially in that first bit of The Waterways, where, unless your platforming is on-point, you have to do multiple loops to get all the treasure in that body of water just to the left of the entrance. There's a part of me that would like to see the little bubble that auto-swaps characters used more prominently for cases where the player's going to see something and know exactly which gal to swap over to, but I can understand that those little moment-to-moment considerations may be something you'd prefer to keep intact. Obviously this is all preference, and, critically, knowing where to go and who to use in repeat playthroughs cuts down a LOT on my nitpicks here and the pace feels enjoyably brisk for it.
I love the CGs for major cutscenes! They do a lot for helping those scenes feel unique. I sometimes wanted some sort of indicator for who's talking, whether that's around the textbox or localized to the CGs. Each of the characters have a clear voice and their lines match whatever they're doing, so telling who is who is still manageable, I just think a little extra help would be nice, since it took me a little time to get all the gals' names straight.
A visual+sound indicator for when abilities are recharged would be nice!
Especially since there are times where it's preferable to have the HUD off.
I found myself wanting the range on Rosa's sword to be little longer, especially since it doesn't deal much damage. The spitting green goo enemies in particular are really hard for her to manage which lead to me walking some distance to swap Izzy in, though maybe that's by intent!
The recovery notes that drop from enemies were a bit tough for me to catch as Wendy or Izzy.
I didn't understand the sibling puzzle until the second time around. The first time, I just guessed - it wasn't until I went back to this area when messing around that I noticed the diagrams corresponding to the ice temple entrance. I did eventually pick it up, though!
It wasn't super clear for me that the little vine boss in The Waterways can only take damage when the eye is open in its first phase, but... also I did get it before long, so that maaay be a bit nitpicky. Especially if figuring that out is supposed to be kind of puzzle-esque.
It was unclear to me whether dying eats the treasure you've saved up? It seemed like that's what happened when I died and continued? If it's supposed to do that, I think that might be a little mean when leaving a level undoes certain progress and uncollects treasure, because I played in such a way that I wanted to go back to get food when things got risky. Also, I think sequence breaking might be possible because keys are uncollected but checkpoints stay active? But you might be into that, that'd be valid
Related to that, I quit the game after dying to the dragon boss in order to avoid losing my cash, which I didn't expect to kick me back to the town (I also did not expect the game to close entirely). Basically, I found myself wanting a little more clarity about the punishment for failure.
Just wanted to stick a note at the end of all that to say that I love the music! Just, in general, but I particularly enjoy the boss theme and the Holy Chambers theme. The music in general is well-composed, well-mixed, and creates a vibe which feels unique to your game, and I look forward to hearing more of it! In general, the sound work feels a lot more polished than many other similarly-scoped games, great job here.
ultra-nitpicks:
If two NPCs are overlapping, ending one's dialogue will start the other's! This fortunately doesn't seem to result in any sort of softlock.
Could the gem display be x/y instead of a countdown? I was mildly alarmed when I noticed my gem count had somehow gone down, only to realize that it's showing the number left, not the number I have. Extreme nitpick, I know, just, the money count is right next to it and that goes up!
Anyway! There's all that. Provided you read everything I had to say there, I want reiterate that the issues I had really were largely nitpicks and that the game is otherwise very polished, to the point that it has this sense of being "solid" that a lot of small games struggle to have. I strongly look forward to the finished game, and I hope that this mass of feedback helps a little!