Honestly, I don't know how you achieved it, but I love and hate this visual novel all at the same time.
I know this sounds controversial at first, but let me explain.
In a very short format, you've packed romance, drama, psychological horror, and also straight-up paranormal horror. You had four main characters to use, and you used them to the fullest extent the short format allowed. You really captured the characters' emotions well.
The plot is simple and short, but full of many ambiguities. You didn't make the mistake of forcing the plot to create a complete story in a short format. Doing so would have stripped the story of its realism, as it would have made the main characters omniscient in this world. Instead, you focused on secrets and bits of information, and it worked brilliantly. Furthermore, this mystery simply made the story more engaging.
I also like some of the scenes where you can feel a sense of unease. There were a few times I didn't understand what was happening, but I knew something disturbing and sinister was happening. And that feeling of not knowing intensified a certain feeling of small shivers.
So why do I hate this game as much as I love it? Because it's a short story. I was truly captivated by the story, and I could see a lot of potential for it to expand. The open-ended ending is brilliant, but it feels like it's screaming at me. The number of puzzles and the lack of better clues raises so many questions that truly demand answers.
I think I'd rate this novella an 8 out of 10. It's simply a very good, well-written short story with the potential for expansion.