First of all, congrats on creating your first build! It's clear from the get-go that you have a pretty ambitious project and I can say the premise is quite exciting!
Good news, even though it's still a prototype that might see a lot of changes, the plot points we got here are interesting. The visuals are just stunning and sell the mood and vibe of the story. The sprite work is pretty well-done too! Assuming this is the direction you're going after, both in the story and the presentation, it's gonna be an interesting VN to keep under the radar.
Before moving on to the not-so-good news, please keep in mind these criticisms are not meant to tear the project down. Instead, keep it as a suggestion so the problems can be addressed way earlier in the development phase.
Also worth noting is the time limit effect (from the game jam), plus the fact that this is your first attempt, which contributes to these issues.
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First one is the UI. Despite the gorgeous artwork put into the build, the UI definitely needs some rework. Some text on the game menu is really straining the eyes to look at due to the color choice. The contrast of the dialogue text with the text box, for me personally, needs more tweaking. It's a visual novel after all, so it's mandatory to keep the readers comfortable going through the game.
Secondly, and this is the most important part, the writing needs major editing. I don't know how much language barrier / translation issues contribute to the problem, but a big chunk of the prose felt dry, in contrast to what I assume was your goal, a more poetic or flowery narration. The conversation between characters felt... unnatural... stiff, almost mechanical and in a way more of a "telling" than actually a "talk between two individuals". Maybe it's because of the MC's "personality", but I don't think that's the case if the three characters got same tendency.
Speaking of "telling", the part where we switch into the NVL mode felt weird at first because I couldn't tell "why is it in NVL now?" Later on, I would assume the decision was because the story switched to telling some sort of history or factual things happening, hence the more loredump-ing vibe. I think it needs some touch to make the transition less jarring.
And in the matter of "jarring", the first time MC's name was displayed my reaction was pretty much "Huh? Is this some kind of bug?" (evidently, that's not the case). It would help greatly if you can address the naming, as simple as giving a line like "Twenty-one. It's the peculiar name that he goes by. The name she chose for this damned yet blessed soul." or something along that line.
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Again, please don't let this comment, or any other one, discourage you. It's a natural part of the learning process.
Wishing you good luck on this project ^^