That Prince Is A Girl: The Princess And Her Grand King

To protect her sister, Emeriel disguises herself as a male servant when both sisters are sold as slaves to the beasts kingdom of Urai. But Emeriel is no ordinary human, she's a rare Syren, the only one who can calm the beast king Daemonikai in his feral state, and they secretly fall for each other. As the jealous Sinai torments her, the ambitious lord Zaiper schemes against her, and the cursed Vladya becomes obsessed with her sister, Emeriel fights to keep her identity hidden while navigating deadly traps. In the end, she breaks the beast-king's curse unexpectedly.