Beneath the Surviving Princess Joyful Facade

"Very good. I shall give you my beloved sister, whom I cherish dearly."

Eric Chladnie, who led the long war to victory against the Shidate Principality, received the mandate to marry a foolish princess as a reward for his triumph.
After the wedding ceremony, Eric returned to the mansion with the newlywed princess. Amidst the princess's eccentric behavior, his father attempts to kill her and his mother advice not to give his heart. Only he, treating her like a child, respects and cherishes her as his wife.
However, who is this woman who, in front of the king, impersonates deeds that she has never done and acts piously?