This was really impressive! It's always a little tough to evaluate something on what is effectively a first chapter, but this jam isn't ranked or anything anyway so I'll just give my thoughts so far.
I'll go ahead and put a spoiler warning here, but from the screenshots provided I... don't know if it's really necessary? I'll get to that.
First, the presentation here is really exceptional. The sprites and backgrounds all look really great and the watercolor style really complements both the architecture and landscapes on display as well as the surreal nature of which those are depicted (e.g., the creative angles upon encountering the house). The couple CGs we get are good too, and I'm glad we get the customary mirror shot to get a good look at the first-person protagonist. With that said, I kinda question why you would use the CG from the literal last scene of the demo in the marketing material (like the store page screenshots)? I feel like you want to save those moments for their reveal in the story. Seeing the buck act outwardly evil to a little fox who looks an awful lot like the fox in one of the other screenshots would really mess up the way the tension builds in the story since you know, and not just get a sinking feeling, that the buck is a bad guy.
Before I get into plot and characters, I also want to call attention to the strengths of both the music and the prose in this VN. This might be some of the most effective use of music I've seen (er, heard, I guess) in the jam and it does a lot to add to the tension and mystique without being overbearing. The organ track in the foyer was a particular highlight. Writing is generally very good here, and you have an excellent grasp on conveying vivid imagery without dipping too deep into purple prose and over-reliance on figures of speech. With that said, I know this is a jam and deadlines are a thing, but this could've used another proofreading run. Besides the egregious "aidou" instead of "adieu" that someone else mentioned on the store page there were a couple other howlers too, like "grandure" instead of "grandeur" and "deocation" instead of "decoration".
In terms of plot, I think this does a (mostly) good job of slowly ramping up the tension. I'm getting a lot of Beauty and the Beast vibes from the setup, with the whole finding-a-mansion-in-the-woods-after-getting-lost-in-the-snow premise as well as the French names and overall aesthetic, and I'm not sure if that's intentional or not. Given the placement of the mansion and its anachronisms I would guess there's something supernatural going on. The name "Sylvain" also seems a bit on the nose, but I also suspect that might not be his real name considering it's conspicuously absent from the Jacques flashback.
Problems thus far:
1) As effective as the understated horror of the second Jacques flashback is (what did Sylvain do to Jacques to condition him to be "the pleasure" after kidnapping him as a child? Eugh) I feel like it's a huge jump forward in escalation from the very slow burn thus far, and I wonder if that might have been showing your hand too early. I know it's a demo and you want that hook to bring people back, but I don't know how that'll affect the pacing going forward.
2) As a silver lining though, I think it might end up addressing the second issue. I know Oscar is desperate for shelter and all and is recovering from near hypothermia, but he does not come off anywhere near as suspicious as he should be, especially after the bath. Hopefully the dream sequences get his ass a bit more in gear, but I would have liked to see him at least a bit more uneasy with everything going on.
Overall though, this was great and I'd really like to see this story continued! I really hope there's a way we can escape and take Jacques and Marie with us but I looked at the images folder and I have my doubts. Really hope those sprites are from nightmare sequences.