Valerian

In the past, in a poor kingdom, a girl was born. The king of that kingdom owed a great debt to another very powerful king, but because the kingdom was impoverished, he could not repay it. So, when the princess was born, he made a very difficult decision: he gave his own daughter to the other king as a way to pay the debt. The girl was taken as a baby to the castle of the other kingdom.
In that castle there was already a prince, the king's only son. When the girl arrived, he was four years old. Over time, she grew up believing that this family was her true family. To her, that king was her father and that queen was her mother.
The years passed.
The prince grew up training to be king: he learned to fight with a sword, ride horses, study strategy, and rule a kingdom. He also knew the truth about the girl since he was ten years old, but he kept it a secret.
The girl, however, was raised like a princess: she learned etiquette, dance, reading, the politics of the kingdom, and how to represent the people.
Time continued to pass.
When the prince was 18 and she was 14, the two were already very close, almost like best friends. He always protected and cared for her.
A few years later, he turned 24 and she turned 20. The princess was officially crowned princess of the kingdom in a grand ceremony. At the same time, the prince was about to be crowned king, because his father was very ill and it was known that he would not live much longer.