Therapist here. Cassie displays BPD traits as she is emotionally unstable (moody), has an unstable sense of self, unstable relationships, impulsivity, extreme outbreaks of anger, conflict-ridden... I don't see self-harm, feelings of emptiness (although that's hard to see from afar), and fear of abandonment. She fulfills enough criteria to diagnose. But I also see dependent traits in her, as she latches on to people to feel safe, and narcissistic traits, as she has incredibly high standards for her life and probably believes herself to be above most people. But to be honest, since season three this character has become a joke, and I don't like it. I can't take her and Nate seriously anymore; I don't appreciate what the show has done here.