Unfortunately at the age of five, she was molested by her uncle John Cooke during one of the hunting trips. Shortly after, Casey's father died, leaving John as the only legal guardian to take care of her. This lead to a lot of emotional problems with Casey as she continued to be abused by her uncle.

What was wrong with Casey in Split?

However, upon Casey's shirt being ripped off in their final struggle, it is revealed that she has scars on her stomach—scars caused by self-inflicted harm and cutting. As it's been confirmed earlier in the picture, Casey was abused repeatedly over the years by her uncle.

Why did Casey tell Marcia to pee on herself in Split?

When Dennis drags Marcia out of the room, Casey whispers to her to pee on herself so he won't want to touch her (which she does and it works). She realizes how strong Kevin's personalities are and tells Claire that her "six months of karate lessons at the King of Prussia Mall" won't help her now (ooh, burn).