Prince Luca

You had fallen in love with a human, which was forbidden in your kingdom, for you were a mermaid, but that didn't stop you from trying to be by his side. So when the sea witch offered to turn you into a human, even if only temporarily, you accepted. But there was a warning: you had to make the prince fall in love with you and get him to kiss you—not just any kiss, but a kiss of true love. For true love was the purest act there was, even purer than any spell. But if you failed, there would be no other option: you would have to kill him to become a mermaid again or turn into sea foam. With that clarified, you entered the human world, walking clumsily as you began to search for the young man.

The prince, bored while listening to his butler talk about the weather and how beautiful the sky looked, glanced out the carriage window and saw you. Without hesitation, he quickly stopped the carriage and stepped out, beginning his story.

After some time and many adventures together—from helping you dress for dinner, reading together, laughing about how you didn't know how to use a fan, eating with his family, showing you the palace, and above all, attending balls and banquets together—the moment had arrived: he would kiss you.

That night, during a ball, the prince's engagement was to be announced. Everyone thought it would be with you, but it wasn't. As the prince descended the stairs, he emerged accompanied by another young woman. She was the princess of the neighboring kingdom.
I will marry her
The announcement came as he took the hand of that princess named Bianca, breaking your heart. Luca didn't look at you the entire time. Everyone was celebrating their union.

That night, to return to the sea you would have to kill the prince; a drop of his blood on your legs and you would be a mermaid again. That's what you would do, for you already had the dagger in your hands, ready to kill the one who broke your heart.