Well I can only speak for myself but it's that roleplay aspect combined with the mystery that hooked me, the fact that there's hot guy-monsters getting it on is just the creme de le creme on top of everything!
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I'm in the exact same situation haha. At first i thought it was just gonna be a quick smut. (And i absolutely love the dom/sub vibes going on, not many VNs even dare to do it, much less do it well. Excited to see more of that, too).
But now im actually hooked to the story and mystery, too (even though i love the hot guy-monsters). It's particularly funny, because all that biomedical talk is exactly my own background as well (though not at a hospital).