Beat the demo, I quite enjoyed myself. What music did you use? It's quite nice. Also sorry for the wall of text, this really is ridiculous, but I swear I don't repeat myself. At least it's comprehensive, I suppose...
Visuals: I like the pixel art, though I first miss-interpreted the Finnegan sprite as a monkey man because his armor was so uniform. Maybe add some contrast between the plates of armor or something? Character and enemy design is weird yet appealing, it really feels like a fever dream brought to life. Which I think is the right vibe given the setting of Purgatory. The home screen being 4:3 is weird since everything else is 16:9. The text during the home screen conversations and a few other areas I forget looks bad(present on main menu, but not as noticeable). It looks like an upscaled pixel font, where it's blurry in places and bulges sometimes despite looking like it should be blocky crisp pixels(see pic bellow). I forget how, but I think there's a relatively simple way of fixing it in Godot. I really enjoy the animated portraits, I miss when they were common.
Gameplay: I really like the bounce back and forth system, it's an ATB bar that doesn't waste UI space, and it effects the characters when they walk over hazards in real time. Took me a bit to get used to, but using wait vs attack vs skill became quite intuitive. While fun, on stage 3+ it became formulaic with how I dealt with targeting enemies. Perks may have helped but I didn't get any(see Bugs section). Stage selection is ok but doesn't have as much impact as a typical slay the spire like, since instead almost all landmarks are battles, with only level rests once (or twice) per stage. You can choose the stage hazard I guess, though I found they were fairly inconsequential, except for the desert that destroys my DPS and makes me question how much damage I can deal in a time frame. That said, I don't think it would be improved necessarily if you did do the standard trappings, like a boon landmark or whatever. Also on the last map got so big that I couldn't see all landmarks and couldn't scroll it, so I got worried it wouldn't scroll up and got railroaded into the lower path. Of the four stats it seems AGI may be the best, since not only do you act faster, but it also allows you more actions per minute, which increases MP generation rate for skills. STR seems fine, I put some points in it for Finnegan. ULT seemed lackluster, I tried speccing Mosca in it and I didn't feel any returns until I started speccing AGI. RES is probably fine, but I didn't touch it since I wanted to be more powerful not more tanky(which would probably help with Mosca not dying so much). Speaking of, I didn't game over at all until the end boss, and in which case only happened because Mosca was targeted and killed before she made the first rebound. It happened twice more after that, so I am very grateful for the save right before the fight.
Story: The setting works well to justify the gameplay, and the bestiary aids with extra details for those who read. I like the narrative of having an overarching goal of leaving Purgatory as well as the mysterious past of our main characters and the rivals they face. The characters having more philosophical talk was nice to have, and it makes sense given it's just a day in the life for them while they think about their position in the strange world. The last story hook was interesting, though in a way pretty obvious since the characters were bringing up points about how weird Purgatory is several times.
Tutorials/Information: I read the HOW book and still had some trouble following along in the beginning. HELLO text was distracting a bit. How my MP got drained in battle always felt random, it should get the same treatment as HP and come out as a floating away text on applicable hits so that I could at least figure which enemy is responsible(it's probably in the Bestiary, but I chose to ignore it since I didn't want to memorize a bunch). Combat actions feel too quick to start, by the time I register a new enemy move is being used it's already over. Having a description of the selected skill would be nice.
Bugs: No perks at all, got to level 7. Though for such a big error you probably released a new build I didn't get in time. When I died in the final fight the battle music kept playing upon returning to base and stayed until I got into the fight again.
Suggestions: I think it would help to display the hotkeys in battle somehow, so the the player remembers they exist(I forgot about D for a long while). HP/MP display with D should probably be a toggle or menu option that stays on forever if one wishes it. Maybe color code STR, AGI, ULT, RES with their respective icon colors when in text form? It would make it even easier to tell what formulas use what at a glance. If you want to be extra fancy, you could also have special colors for terms like SLOWS, XX%, X_SECONDS, etc. Several times I got the same encounter (including formation)back to back, might be worth adding an anti repeat method.