How is Casey's history related to the movie Split (2017)? Casey has an uncle who rapes her since she was three. I can only imagine what she must have gone through, but somehow it feels like this is unrelated to the overall story.
Because Casey was abused by her uncle, she reacts differently from the other girls in captivity. She doesn't try to resist or escape (at first) but doesn't submit either. She's psychologically stronger than her two friends. The whole point of her story is to make a clear distinction between broken and not-broken characters.
When the Beast prepares to attack her and sees her scars he stops and recognizes her as one of his own, broken, thus more evolved.
Quoting The New Yorker,
Shyamalan sets out the nature of that decisive difference between the broken and the rest in scenes involving another character—Kevin’s therapist, Karen (Betty Buckley), whom he sees often in the course of the action.