before the murder, she was (and still is) a sweet person who never liked causing trouble and generally stayed on everyone's good side. however behind closed doors, she was a really troubled person and has had many suicide attempts before being sent to an institute by her family. she was troubled because she had this skewed idea that no one loved or cared about her. so she stayed there until she was eighteen and discharged. so when she met the "love of her life" she finally believed she had a purpose. unfortunately when the abuse started it really messed her up and therefore she snapped. in blind rage she stabbed him to death. anyway she went to jail but got sent back to the institute for another suicide attempt.
even though she's very nice, she has trust issues. so when someone shows her kindness she believes they're faking it but gives them a chance before jumping to conclusions. but i do see rose and cass being friends and rose being concerned about cass and how she does relationships.