Unmasked: My Brother's Fiancée Is Freaking Out

Coral, a human girl who breathes underwater, becomes a full mermaid only at 18. She planned to yield the Sea King's throne to her brother and live as a human. But his fiancée Sirena, a sea demon who eats humans to stay beautiful, mistakes Coral for a human mistress and tortures her. Coral tries to save her brother and uncovers a greater conspiracy.