It's a difficult coping mechanism. She behaves the way he wants her to so he won't hurt her. The public views it as them being very intimate because Cesare has incestuous feelings for Ruby but behind closed doors, he's someone very controlling over what she can and can't do. She tries to appeal to him to prevent him becoming abusive with his possessive feelings.
Like the commenter above said, it’s a defense/coping mechanism. She is playing him so he doesn’t hurt her. Spoilers:
Later on when Izek finds out, he feels enormous amounts of guilt he never saw this coming. He mentions that’s she acts so bright and happy around Cesar and that’s why he never noticed. Ruby responds by saying that her whole act was centered around Cesar’s volatile personality so her acting was at its best when she was with him (around 3 years of acting since she entered Ruby’s body I think).
So yeah it’s all to minimize the amount of abuse she receives from him. She learned how to act and what to say all to protect herself. She couldn’t run away because of Cesar’s influence. She couldn’t go to her father because he would also beat her. Enzo is the nicest too her but but he wouldn’t go against his family to help her. So she was basically stuck learning how to minimize all abuse by just giving them what they want. A happy doll like girl that LOVES her family and will listen no matter what. Obviously our Ruby isn’t like that but the previous Ruby was.
I'm confused about the relationship between ruby and Cesare? I don't remember why she is pretending to be normal around him? Can someone explain it to me?