I believe that's how she actually is? She didn't save Cassis because she felt sorry for him. She did say that she only wanted to save him for her own survival and interests.
The only reason she still has some feelings is because of Asil but it was also his death that caused her to become the Roxana before she met Cassis. She became more human because Cassis treats her kindly (yeah, it happened much later once they started trusting each other) despite them having a "master-toy" relationship. You could tell Roxana grew attached to him before he left the Agriche castle.
But I wouldn't call her cruel facade as her real personality because she's only doing it for survival unlike character like Jeremy, Charlotte, and Maria who seek out violence and chaos.
Sure, she's cold and calculating, but she seems more interested in living her life without having to worry about her psycho family rather than the power struggle to be the successor.
Is it bad that I wish Roxanne's personality she shows to her father was her real personality and instead she learns compassion from Cassis's?
Also, it's really neat that the Agriche kids are really stereotypical. Dion, the oldest, wants affection and to be babied. Jeremy, the middle child, just wants attention. And Charlotte, is a typical youngest that wants everything.