Looked into this after finding a cheap copy at a used bookstore. 'Hellions' is a standalone novel by Chloe Gong, a twist on 'The Little Mermaid' set in 1920s Shanghai. She's best known for her 'These Violent Delights' series, so it's an easy mix-up, but this one's a solo act. I appreciated that because I was craving a complete story without committing to a trilogy.
It does have thematic links to her other works, being a fairytale retelling in a similar historical-fantasy vein. If you loved the atmosphere of her series, you'll probably like this too, but you won't be missing any crucial background. I read it in two sittings and felt the ending wrapped things up definitively.