Past
For Rose, the past began 11,000 years before the events of the game during a time considered legend. Not only did she experience those legends, she was a main participant and contributor to the current state of the world and as such she took great responsibility in protecting it.
Childhood
Rose's childhood was never revealed during the game. She was born into an era of subjugation where Winglies dominated all other species and controlled the gods. Humans were little more than slaves and it was likely her childhood that molded Rose into a revoutionary.
Standing On Her Own
At some point, Rose met Zieg. He was with her when she defeated the Dark Dragon and became the Darkness Dragoon. The timeframe of when that happened was not revealed. Since Diaz had seven Dragoons when he declared war in Fort Magrad, Rose most have become a Dragoon before or shortly after she met Diaz. Zieg liked introduced her.
The Dragon Campaign was a long war. Rose stopped aging at 26 just after the pinnicle battle so she probably joined the fight in her early twenties. During this time Rose was an idealist who believed revolutionary rhetoric such as, "...the arrow would not only shear the darkness that covers the earth. But also shear their corrupt hearts which were accustomed to being ruled."
Rose believed in equality of the species, that there was more to life than slavery and death. She joined with like-minded individuals and became an unstoppable force. The right hand of Diaz, they fought against impossible odds destroying flying cities and reducing an empire to nothing. Kadessa was the definitive battle during the Dragon Campaign. It was this city that saw the death of Melbu Frahma, the Dragoons and the Winglies' empire. Rose watched her lover and friends die in that battle. She survived a turbulent generation, helped shape the world and lost everything. It didn't seem grand or heroic anymore.
Aftermath
The battle that brought peace also brought about the destruction of the crystal sphere that contain the Virage Embryo and created a cycle of death. "Every 108 years, when the Moon That Never Sets glows red a moon child will bless the world with holy bliss." Rose was one of a select few who knew what that truly meant.
As a last defence Charle, the sister of the late Wingly Emperor, told Rose the story behind the legend. Rose was told that the Moon Child's blessing was actually destruction. If allowed to enter the Moon that Never Sets, the God of Destruction would be born and the world would be destroyed. Rose was sent every 108 years to kill the Moon Child and all those infected by it. Rose grew to be known as the Black Monster and became object of hatred and revenge. Those long years reshaped the revolutionary into a cold emotionless woman who's insane duty was the only thing that could keep her sanity.