Lothiriel/Eomer
Based on that whole Eros and Psyche story where Eros is like some invisible nighttime lover or something (but only the invisible nightttime lover part; no other previous or subsequent events). Lothiriel never discovers who, unlike Psyche, and of course nothing bad comes of this like it did in the myth. Anyways, Psyche=Lothiriel and Eros=Eomer. Pre-established relationship. Dol Amroth setting. One year after Eomer ascends to the throne (and ya know they didn't marry until two years later). Sorry about the specificness ;-D