Personality
Rose went though two distinct events that dramatically shaped her. Her personality before the conclusion of the Dragon Campaign was different that her after being the Black Monster for years which was different than her taking on the God of Destruction alongside Dart and their friends. Rose's personality revealed a strong heroine who was capable by herself but even stronger with allies.
Rose also avoided the usual fate of heroines, needing to be saved by the hero and falling in love with him. Instead Rose quickly realised her feelings for Dart weren't for Dart but rather her dead lover of whom Dart resembled. Rose needed companionship but she did not need a hero and this made her a strong role model for anyone. Rose saved Dart and Dart saved Rose but they didn't have to be lovers to do it.
Revolutionary
Rose grew in an age of turmoil and change. A young woman, she had nothing holding her back. Not having to put food on the table or raise children, Rose was free to pursue the lofty ideals of youth.
Before the Dragon Campaign, Rose was idealistic, revolutionary and naive. She believed that humans were just in fighting against the Winglies. She fought for a new world and her ideals. Equality, tolerance, acceptance, peace. These were all things that Rose believed and fought in. She joined Emperor Diaz's army, fell in love with a charismatic and handsome Zieg, and tore apart a tyrant's empire.
However at the end, she was faced with tremendous loss. Her friends and lover were all killed, her vassal dragon eventually fell into bloodlust and she was forced to kill him, and above all she had to take on an insane quest to fix an innocent mistake. Rose was given the task of killing an innocent child every 108 years along with whoever was in contact with the child. Rose went from being a revolutionary and fighting for what was right to losing everything and killing babies to stop the destruction of the world that her campaign started. Anyone else would have gone crazy. Rose accepted it and adapted to it.
Fixing Mistakes
After the Dragon Campaign, Rose was able to look back at those events though the eyes of a survivor. She lost the idealism of the revolution. There was no nobility in the Dragon Campaign, only slaughter and death. She didn't just bring happiness to those without. She took happiness from those who had. Rose lost her naivety and innocence. War brought understanding that her dreams on built on sacrifice. Part of Shana is what Rose was long ago and part of Rose wants to be that way again. Innocent and oblivious.
Her personality reflected this change from revolutionary to counter-revolutionary. Instead of fighting for change, she fought for stablility. In order to prevent the deaths of her friends of being in vain and to prevent their actions from causing the destruction of Endiness, Rose closed off herself and refused to sit in self-pity. She had a mission to do and she would not let her emotions or feelings get in the way. The Rose who the party encounted is very different from the Rose Zieg knew. She showed a strong jaded streak of someone who has lived too long without hapiness. Rose was little more than a shell of her former self, someone who had even forgotten how to smile.
Reawakening
When Dart first encountered Rose, she was a no-nonsence, straight to the point woman. She was cold and sharp with no interest in trivial things. Her next mission was years away and really, all she was doing was killing time. The Darkness Dragoon Spirit's reaction to Dart had peaked her interest because there should have never been a reaction. Rose wanted to find out who Dart was, she had little interest in the Serdio War or his revenge. However their travels together revealed that the Dragoons were gathering again, Virage still lived and someone was trying to collect the Divine Moon Objects.
Rumor
Though I haven't verified this, I have heard an interesting bit of information regarding Rose's past and present demenor in respect to the Japanese script. During the flashback with Rose where she defeated the Darkness Dragon and Michael was born, Rose used the same type of language as Meru did when she was talking about Savan's robot 'babies'. This was apparently different that the language she used at present. I'm guessing Rose was using more womanly and polite language in the flashback.