I think it's going to turn out that Hikoichi was right all along. Nayu was wearing Shion's mask trying to re-enact the same scene from the game that he was thinking of.
She figured than rather than betting on Hikoichi being able to spot her face in the dark out of thousands of girls with similar build and hair type, which we see is giving him trouble (he apologizes to one girl and gets run off by another's companion), she'd rather bank on him spotting the mask that looks exactly like the character's they both love, standing in a spot outside of the crowds like the one she was standing. Not saying that's objectively correct of her to do, but poetically, it's quite lovely.