I rather liked Kevin and Casey's complicated connection. She's been abused all her life, and her experience with Kevin however horrific helped her. It gave her the courage to speak out and stop her torturous abuser and finally begin to heal. Now Casey wants to return the favor and help Kevin heal, understanding Kevin is also sick and a sad product of years at the hands of an abuser.
On a side note, the Mental Hospital used in Glass was really a closed down Mental Hospital thirty minutes away from me. So last Summer I visited it. It is gorgeous, yet creepy property.