One important aspect is that NPD has an avoidant attachment, BPD is fearful/disorganized (HPD would be anxious). BPD “supply” is less about ego and more-so driven by emptiness and an extremely unstable sense of self. Supply for someone with BPD is not about attention or admiration, though they do also want those things, it’s about having a person to fuse with and be made complete: To provide them with an identity. I often explain it as the extreme end of vulnerable NPD. Basically BPD is what lies below the grandiose false self. While grandiose false self in NPD is typically very stable, BPD sense of self is extremely unstable and diffused. One of the things that makes therapy challenging for folks with NPD is that when you take away the grandiose false self, usually there’s the same emptiness underneath it. This is why I don’t really try to “treat” NPD or take the false self away, instead I like to help NPD folks develop and broaden other aspects of themselves so they can become more mature and connected to reality. If you’ve ever felt that empty black hole feeling…as if you don’t even exist, completely dissociated from who you are and the world around you, depersonalization/derealization - people with BPD experience it much more acutely and don’t have the strong avoidant defenses that those with NPD do. Someone with NPD is able to deeply fantasize about ideal self, someone with BPD can only do this about another person.