Damon

A bipolar relationship? Bipolar disorder is a condition and the word can't be used to accurately describe anything else no matter how often people try. It is never a good idea to use a mental disorder to describe anything else, but if you insist, borderline personality disorder would be a closer fit. Whoever started using mental disorders to try to describe anything else started a disturbing trend of inaccuracy, misinformation and stereotypification that can't slither into oblivion soon enough.

But I will agree about her overall instability. Her connection with her brother struck me as just a supernatural version of the very real connection that many completely human twins share, my mother and her sister among them. I didn't think it was strange or unnatural considering what they were. She had some affection for Damon, but I think a lot of her behavior was a front to keep him from knowing how she really felt. Until she saw it for herself, she did not want to believe or admit her twin was incapable of love in any form, understandable considering he was her twin. She wanted to think he at least felt some kind of affection for her. Also acknowledging his complete inhumanity would force her too see herself as less human which could force her to face things she would rather ignore.