For those less familiar with npd/bpd - at the core of both (and codependency to a certain extent) lies inability to self-regulate emotions (especially difficult emotions like shame), that makes people with these PDs use those around them as "human pacifiers" that must regulate them in some way. In case of NPD by establishing superiority of the narcissist, in case of BPD via a fantasy of the other being a "perfect rescuer/knight in shining armour" who will save them from their own internal shame-based turmoil (and via seeing oneself as this knight/martyr saving a lost soul in case of codependency) . When a real human (obviously, eventually) fails to live up to the fantasy, they become "the issue" and are used to push the blame for unregulated feelings and behaviors (caused by shame that goes out of control without external soothing), until a new "fantasy person" is found, repeating the cycle.

Honestly, that's where I would draw the line between these two PDs and other conditions like ahdh and asd - is the behavior motivated by the frantic attempts to escape shame and shift blame?

Some anecdotally observed pattern suggests that many codependants in relationship with BPDs are either diagnosed with ADHD/ASD or suspect that they might have it, not sure what the causal link is.