What exactly was The One Below? It doesn’t seem to have been the actual Devil; more like a sea deity à la the Japanese Sea Dragon King.
How come sometimes it seemed to have all kind of supernatural powers (summoning storms, granting immortality, turning into a small turtle, etc.) but other times it was just a giant turtle that couldn't take a harpoon line off its shell?
What exactly was Jennie’s deal with it? What does a sea monster need a human companion for? And why did it allow her to romance Magnus?
Apparently she lured men to drowning, like a Siren.
Romancing Magnus might be a way for her to endure her existence.