I think that the game is pretty good, don't know if I finished the demo though (got stuck whenever both of the exits to get back to the spawn was blocked, which weirdly only happened when you took the non-merman route; so, I'm guessing that is the end for the demo)
I wrote about a page length of suggestions/bugs/stuff that happened that I don't know if you want in the game, but I can save all of that for later and just talk about how the game felt.
It felt pretty good, felt a bit too fast walking actually (you can trimp ramps/stairs if you strafe and walk forward, I think that is because you made it too where both give you speed normally instead of halving both of the speed when you press both of the buttons), and I like how crouching seems to make them not notice you as much. Sometimes I tried to swap weapons while prone, and it seemed delayed (and it also seems like you have to swap a gun to a gun and then press H to actually get out your melee, which seems like a bug) I also had to play this game at the lowest settings to get it to play smoothly on my bad laptop along with whenever a prompt appeared it seemed to slow down quite a bit (when I tried to play at higher settings). Some of the audio gets cut off early and is really loud (example, the hound howls nearly jumped my seat the first time I heard it), gets triggered at times it shouldn't (mainly just the shop/treasure door at the hub, you can hit the frame of the door and get the fwoosh sound but don't teleport), or doesn't sound like the correct sound for it (when you pick up health it seems like the protag gets hurt, climbing the vines sound to metallic and not crunchy enough, the grunt of jumping is pretty repetitive (which could be solved by randomizing the pitch a bit))
Having the vines be something you can go through I like (because it can open up secret areas because you could hide stuff behind them), and I feel like it being space is fine, I got pretty used to it the second time I climbed the vines, however what BoJustBo said about jumping late is sadly a tad true, but then I just started jumping earlier and it turned out fine.
Now one thing that I think that you should really look at/work on is the saving system, because it doesn't seem to remember if you killed all the enemies in a room before hand if you are not in that room when you save (example: the top entrance to the merman room, you can shoot the merman and save from the top perch without going in the room, but when you load they are there again, as well as the items! (which I promptly used to not die all the time by save scumming next to them))
The animations, landscape, and art is really nice (however the merman with the trident walk is pretty weird cause it just seems like they are hovering towards you); the enemies being really fast kinda isn't (the big guy with the chain is a lot faster than you would think, being a lumbering giant and all, and the merman are pretty fast fish out of water), but that is probably my inexperience with shooters talking there (I really only play TF2).
But Rupture is quite a lovely game from what I seen, and my offer still stands to set up and run a discord for this game for you if you want (I already done one for the guy who made Sand: A Superfluous Game) cause I luckily still have the time too, and I hope you have a great time adding on to this game!