no idea who it is as of now. Though I do have some theories. (Spoilers)
It could be Fawn after the retrieval from the upper crust of new arc city. The only reason I think it MIGHT be her is because you move into the same condo as her once you talk to her next. Though it could also be Rinny, but she hasn’t really said anything about it? Mostly because in most media, succubi are seen as dream demons. Other than that, I have no clue as to whom it would be referring to.