Strange that some of commentators didn't noticed big word [Demo] written in the thread's name, and criticize it like it's full release. For concept build it's good and can be finished.
About problems: people already mentioned darkness, and I will say that it's not completely bad, it's atmospheric and helps to hide places which not needed to be rendered, but MC location and direction she should go always must be lighted.
I played it on old laptop and noticed temperature rise in some locations of game despite it being fully dark and hardly visible MC, so You should start to think about optimization and memory leak problems from the beginning of project to prevent this from piling up and burying Your hard work, cause if laptop can run Total War and Civilization it should not have problems with side-scroller.
About things to improve: add on-screen prompt when MC can change locations and button for it different from space key, camera moving lower or higher if MC needs to move there, Demo has some places with blind spots where MC dies when jumping not seeing where, different keys for skipping text and for confirming choice to prevent wrong answers, closer MC view in non-dangerous platformer locations, graphical options in menu.
About things I liked: Kiera model and animation, dialogue options, big thick nipples of Mayoress, Darkest Dungeon vibes.
About things to add: I don't know how grim You want the game to be, but I think You need to change spikes on portals, I mean she can't leave this place and returns to checkpoint every time she dies, so why not explain it plotwise and show that she cannot die to escape this place cause it wants her to eternally lose (and be fucked).
Also You can mention that God of Game changes trial for every contestant knowing their weaknesses and that is why Hero sees scary cave with monsters and Kiera meets thick juicy fertile tentacles.
Don't know if You want to add inventory system with items but it can help to explain Kiera's motivation to do optional missions and strange requests.
Personal wishes: here I mention stuff I would like to see added. Pregnancy system, but when adding something You need to think what it adds to gameplay, what motivates Kiera, not player, to get pregnant with some repulsive creature's spawn, so if You'll add inventory system Kiera can have a chance to get some useful items that can help her in trials so she will have a choice to be mother for profit or try beat challenges a hard way, quests for bringing spawn of specific monster to somebody (to Mayoress for her private "collection", looks like she is into it judging by her castle's design, or maybe she wants to train Kiera into personal breeder for her "pets" by giving her this strange missions, who knows?).
Lactation which can be triggered by pregnancy or monsters' actions, parasites that attach to her body (or inside of it), puzzles which can be solved two different ways (smart way by doing challenges and solving riddles or slut way by appeasing God by making offerings to his altars (parts of herself, her "children" or her milk)), unlockable costumes like tentacle suit that sucking her breasts and keeping her pregnant.
And one question, picture with Mayoress in bedroom copies Goya's painting "Saturn Devouring His Son", hope the game won't have guro and cannibalism, Kiera should give birth to hundreds of various disgusting creatures and be happy cause she helps endangered species, porn games are about fucking, not about death of sexy girls.
Sorry for writing so much words, I've read negative comments though You just asked people to give honest review, so I decided to give comprehensive opinion. Most of things I described are silly or too difficult to do, just keep doing what You like and don't get discouraged, good luck and stay safe.