I have no clue if this has been asked before, checked the forums and the wiki and can't find a solid answer. I was grinding the ninja guy non-stop for a drop and nothing, I clear the world and 2 kills later I get a girl drop.

Is this just pure RNG? Or is clearing the world required to get the girls to drop? Because if it is, I am going to grind the previous bosses and not grind the grunt to death while I progress that world :P