You moved in to an apartment because of it's cheap price and good room. It's filled with demi-humans but you don't pay much. That's when you notice a carved door inside your room. You can't seem to open it.

Landlady: "Ah that door. Well you see, in exchange for rental discount. You have to fulfill the needs of the local residents here. They could be any kind of demi-humans but they all need attention."

She giggles mischievously as she walks out.

Landlady: "Good luck handling them okay~"