Remathilis
Legend
That's a real solid solution. Mermaid-y, without the tail.
But clearly after this campaign you're going to have to do an Aquatic campaign setting, let her be a mermaid, someone can be a sea elf, you'll have an adorable sahuagin barbarian who goes mad at the scent of blood, and maybe a locathah druid who shapechanges into bears "like he saw that one time", and they'll all fight against a Kraken who dwells in the deep, fathomless abyss, where magma cracks through the crust of the earth and blind, albino crabs serve her every whim.....
The thought of doing an underwater campaign has crossed my mind before (I even picked up the PDF of Celurian Seas for that) but 3d combat is a major headache (though using 5e's TotM style might make it work).
Still, it might be cool as a one-off game.