yup! this is more of an AU where his darling (also known as Mitsuki) in other games died/they never met. And instead the MC is someone Tantei knew in high school but was never close with, and considers as enemies when they grow up.
And you're correct! The MC of this game is Riawa, a doctor who also has her own darling in the original world. But I just wanted to explore the dynamic between Tantei and her further with this game :D
And Tantei would technically treats everyone like how he treats the MC too in normal circumstances.