Rosalie was a normal student but when Gallia fell she went to Britannia on a ferry, where she went into an aviation school. After she became Pilot Officer she became a member of the Free Gallia Air Force, but she didn't often get battle duties. Instead she was in charge of the supply transportation.
When she was on a transporting duty she came to Tsaritsyn, where fierce battles continued. She was transporting Striker Units there but instead of continuing she stayed and volunteered for the defensive battle. The army in charge of the defense answered to her request by establishing the Normandie-Niemen Regiment, which was composed of Witches from Gallia who wished to fight in Orussia.
She distinguished herself in the defensive battles and was promoted to Flying Officer, she simultaneously became the youngest company commander. She caught the attention of Wing Commander Safonov during those battles and was later scouted into the 503rd JFW.
Her ace archetype is Roland de la Poype.