I have yet to get a single girl from any of the bosses. I've heard that it's completely random when it happens, but after seeing someone mention that they got all the girls from a certain boss and could finally move on, I got curious.

Do you need to get all the girls from one boss before the girls held by the next boss will drop? Since I don't have any boss drop girls, would I have to grind Dark Lord for his girls before any other boss would drop girls? Or is this not a factor?