Regin Ester Loire Bastien do Charles is the Crown Princess of Brune Kingdom. She lived as a boy because women had very few possibilities of ascending to the throne, so only a few knew her true gender. She was reported dead after the Battle of the Dinant Plains at the beginning of the story, but in reality she survived with the help of a few servants and went into hiding. While Thenardier and Ganelon suspected she was alive, they took advantage of the King's frail health to do as they pleased. She was found by Tigre when he saved her from soldiers of the Mouzinel army. She revealed her true identity to Tigre, Ellen, Mira, Rim and Marthus after the battle was over, asking for their help to regain the throne that is rightfully hers. She once met Tigre when both were very young and he hunted a bird for her, hence why she began to trust him and begged him for lending her his power. She seems to also have feelings for Tigre. Regin also becomes a good friend of Titta, after both began talking about Tigre. After the civil war she is reunited with her father, King Faron, recognizing her as his heir. She agrees on a temporary alliance with Zhcted for three years, giving Tigre's custody to Ellen while she strengthens her position. After her father finally passes away peacefully, she becomes the first Queen to rule the Kingdom of Brune.

(Source: Wikipedia)