This is a masterpiece haha. Took me three playthroughs to figure out how to unlock the ending I wanted (Petra), but was well worth it.
Your writing is really good, you have a pretty nice sense of humor, got me to laugh out loud a couple of times. I think you did a great job of making each girl feel like distinct characters, and the move of taking three and giving them different starting roles than the others was a really good call. Would have been nice if the Luxiem guys had more than one and done cameos, but their inclussion alone is praise-worthy. The use of sprites is really skillfull, and I love your sfx choices. Backgrounds were all nice also. The Petra piano scene deserves its own mention and maybe an award, delightfully erotic.
If I had to name weak-points, one would be the reveal of who the stalker is. It's never conveyed, either by monologue or dialogue. The cafe scene ends, and you are left feeling, "well, guess she was it." On a second playthrough, it is nicely foreshadowed in her introduction, and MC seems to already have an inkling of the truth, but still, when the moment comes, there's no confirmation. A line of thought or a simple back and forth would have sufficed, so it stands out to me as an "unforced error". Another scene that's not bad per se, but did feel weird to me, was the drink-strip quiz game. And the reason that it felt weird was because, as far as I could tell, it's the only "segment" during the entire common route where you don't get to make a choice (unless not having Selen's route active ends the scene early). I saw both the pool and piano scene choices without Rosemi's route active though, so I don't think that's the explanation, and it's feel weird, specially for such a long scene (dealing with a game that's all about outcomes no less, seems like a missed opportunity for a really engaging situation for the player).
Definitely would be interested in seeing a follow up scenario, either with the rest of NIJI EN, with this same cast in a different context, or with members of a different V-tuber group. But narrative fan-content is something V-tubers certainly could use more of, and it's amazing that you created such a strong offering as your first, would love to see another by all means.