Which fast-acting novel's female protagonist dressed as a beast and male protagonist was a ferocious mermaid?

I don't know of any female protagonist in a fast-acting novel who dressed up as a beast, but I can tell you some information about mermaids. Mermaids were a type of mammalia that lived in the sea. They had beautiful appearances and great strength. On the other hand, the ferocious mermaids were relatively rare and terrifying existences. They often killed for a living. If you want to know more about mermaids, I can provide you with more help.