From what I have managed to gather, this one is part of a series that features a girl and a succubus that acts as a sort of living skinsuit. Originally, the succubus took over the girl, controlling her body while she watched what the succubus did. While she disliked it at first, eventually she wore the succubus more willingly. Now this seems to be after one session where all of a sudden they have switched roles where the girl has become the succubus and the demon has become a normal girl which was not part of the plan, hence their surprise. At least, that is what I can understand of the small nuggets of info I have found.