some ways to write an average joe bpd/npd character who is at least moderately self-aware of their symptoms, from an average joe personality disorder haver:
give them an actual personality outside of their disorder. hobbies, ideals, aspirations, bonds, life experiences. the disorder affects how they navigate around those things in their daily life, not what life they live
you can practice this by taking a cast of characters from a work and thinking about what they would each be like with the same disorder and how their respective contexts would inform their experiences. they're not going to be carbon copies of each other
they may mostly internalize their pain, mood swings, and distorted thinking rather than lashing out at every opportunity. don't make them an explosive cartoon character by default
when triggered, they might suddenly go cold and withdraw (whether they do this out of a sense of self-importance or mindfulness because they know they're triggered and don't want to cause problems depends on the character and the severity of the situation). they may blow off steam by crying in the bathroom or fantasizing intensely for a while, or any number of other things
they could take measures to protect themselves from spiraling, like journaling, keeping helpful reminders and self-soothing materials on hand, or like, letting out their freak in a private safe space. one thing i like to do is exaggerate my thoughts when i'm feeling entitled, like i'm a clown performing. it makes me laugh and i get over it
they'll be spending a LOT of their time actively fighting against a hair-trigger fight or flight response, always checking and second-guessing themselves. for some people, this actually makes them more emotionally intelligent and caring as a result
at the same time, it's difficult to live with the fact that you can never fully trust your own judgment and thinking. this opens you up to potential manipulation and paranoia, which is something your character might be anxious about. they might struggle with figuring out what's really "right", and where the line is between self-love and selfishness
for this reason they might need "reality checks", but only from safe and trusted confidants. personality disorder or not, people don't respond well to being told they're delusional to their face (especially when it's done to invalidate or hurt them rather than help them)
if it's a bad spiral they might lash out in the sense of rationalizing why they're definitely right and you're definitely wrong, or vice versa they're the worst person on earth, and wallowing in self-pity about it. but again, this isn't vastly different from ptsd, anxiety, or autistic meltdowns. they just struggle with this particular issue more often and intensely than most
they may generally come off as detached or with their head in the clouds, particularly when others don't meet their standards (which are based on their insecurities - for example, your character only feels secure around people who share their interests because in their mind this makes them less likely to judge them). that doesn't automatically equate to "bully with a superiority complex". think about how people perceive autism and it's much the same
they may struggle with comparing and defining themselves in relation to others constantly and habitually change themselves in order to fit in, or simply because it's hard to grasp their own sense of self without a template. this may be a form of masking or they could just be gamifying the hell out of being changed for the better (or worse?) by their friends
they may have few genuine or deep connections, which means they could be deeply protective of few they have and be afraid of losing them - potentially pouring in most of their energy into those select few relationships, which sabotages other connections
most of this is unintentional and your character may be willing to adjust to accommodate others' comfort levels, but this can also impact the character's self-esteem as they feel they're always fucking up simple things and failing to meet expectations
don't forget the positives to their experiences as well! they could be very passionate and caring, as mentioned previously, or deeply principled even if it's in ways that make little sense (i like to point at reigen from mp100 as an example. change people's lives for the better with the power of Lying!) it's a nuanced experience is all i'm saying
and on that note, some characters i read as mostly-positive and well-written portrayals of npd: reigen and teru from mob psycho 100, papyrus from undertale. not much but for what little "rep" there is, it's pretty great!
this doesn't perfectly represent everyone with bpd/npd, but these are things i and my friends experience in daily life that i tend to incorporate into bpd/npd characters. i hope this helps writers destigmatize the disorders and have a useful starting point for writing these conditions with more nuance and respect <3
