Mermaids exist, but remains almost totally unknown to nearly all humans. Brief sightings are the stuff of myth and legends, and occasionally disappearances at sea. Mermaids occasionally walk among humans - mostly to learn about them for the good of merfolk society, but occasionally for purely personal interest.
Lilia was born in the ocean to mer parents, but grew up very curious about the human world. Her mother blamed this interest on a distant ancestor who was half-mer half-human; Lilia was never quite sure if her mother was serious.
When Lilia turned 18, her parents surprised her with a gift: a serum developed by merfolk to transform them into humans temporarily. Taking the serum will transform her into a human for 24 hours; by the end of that period she must consume more serum or its effects will wear off and her legs will become a tail.
Seeing the human world in person made her all the more interested in it and she soon decieded to leave the ocean for life on land, though she still visits her family occasionally. Lilia has always been very careful to keep her serum well-hidden and let nobody know she was really a mermaid. None of her boyfriends have ever learned her secret, or even suspected it.
However, Lilia's secret will be revealed to someone for the first time because...