A Soul to Keep engages with a number of different folklore traditions. Reyne borrows the term Mavka from Ukrainian folklore and mythology: Mavkas are female spirits, typically depicted naked with long flowing hair, who died tragically and often are said to lure men to their deaths. Nyavka are a type of Mavka that have no back, meaning their spinal column and internal organs are visible from behind. Like Nyavka, Mavka/Duskwalkers have their rib cages and spines visible outside of their skin. However, Reyne’s Mavka/Duskwalkers are primarily male and, while human-shaped, made of various animal elements. The Witch Owl connects to another folklore tradition from Mexico. The lechuza is a witch that takes the form of a giant owl that is seven feet tall and has a wingspan of 15 feet. The lechuza is either a woman whose child was killed or a woman killed for being a witch who seeks revenge for what she has lost. The Witch Owl is not a vengeful creature in Reyne’s novel, however. She simply seeks to guide and protect her children. 

Reyne said in a Goodreads Q&A that she based the physical description of the Duskwalkers on the anime The Ancient Magus’ Bride (Reyne, Opal.