When Oscar was born she was delighted and commented how she looked like a little princess because of her beauty. Of course Oscar's father was upset, but when he declared in front of her and Oscar's sister that he will raise her as a boy and make her the heir to his military legacy, Grand Mere was horrified as Oscar's life will be in danger from the difficult positions she will be put as a solider.
Grand Mere always wanted Oscar to do girly things from time to time but she knew Oscar's father would never allow it, and Grand Mere hates it when he gets abusive towards Oscar, like when she didn't want to be in the Royal Guard or for minor things and often tries to convince Oscar not to do anything against the queen or her father because of it. Grand Mere however scolded General de Jarjayes for raising Oscar as a boy when she got hurt one time, but she also shows fear of him as she knows what he would do for the sake of his reputation, the crown and for nobility.
When Oscar fell in love with Fersen, she asked Grand Mere to help her dress up for a ball and it made her happy to see Oscar dress as a woman for once in her life, however the happiness was cut short when Oscar got her heart broken and left the Royal Guard for the French army. Then one night after Oscar's father was told by the other noble's to do something about Oscar for going against the crown and helping Robespierre and his followers into the tennis court, Grand Mere was horrified that General Jarjayes planned to kill Oscar as a mercy kill from the Noble's wrath and then end his own life so Oscar wouldn't die alone. Grand Mere begged him not to but he told her he must, however he was stopped by her grandson, Andre, who confessed that he loves Oscar and wants to marry her, Grand Mere cried as it would enrage Oscar's father but Andre tells him that's why they must fight the Revolution so men like Andre can be equal to General Jarjayes and so he and Oscar can be together and end repression, leaving the General in tears as he realizes Andre was right and that his daughter had been right about the Nobility all along and decided to fight against the Nobility himself.
When Oscar and Andre left, Grand Mere finds a letter from Oscar telling her father she thanks him for raising a daughter like her, shocking and devastating them both that they will never see Oscar again.