Some tips for writing a character with actual Narcissistic Personality Disorder (these are based off my experience so it doesn't apply to everyone)
-insecurity is a huge thing in NPD, so being competitive in things they enjoy even if the thing itself isn't directly competitive (like drawing or acting) and not meeting their own expectations of being the best (even if they're praised by others) can make them angry, defensive or depressed
-a lot of people with NPD mask, staying humble even while having a big ego just to keep a good reputation (though some like having a negative reputation, being seen as evil or a villian, but it doesn't apply to all) so they'll pretend to be nice even if they view others as below them just to seem nice or to feel like a good person
-manipulation also tends to be pretty common, whether it's consciously or not, things like, guilt tripping when they don't get what they want, having a victim complex or lying about things for sympathy or to make themselves look better, sometimes it's on purpose, sometimes it's just an unconscious response that they don't even realize is actually manipulation
-god complex, savior complex, hero complex, sometimes they'll do something just to seem like a better person even if they don't actually care, like comforting someone even if they have no empathy or sympathy for the person or situation, to be the hero or savior to them and keep a good reputation of being a "nice person"
-an intense need for attention and needing to be seen how they want to be seen, like if a character thinks of themselves as cool, mysterious and intimidating, they'd get angry or annoyed if they're not viewed that way by others and will try to do more and act differently to be viewed in that way
-sometimes having very transactional relationships, keeping people around that they hate or dislike for whatever they can offer, like if another character is popular, they might hate them but they stick around to keep the popularity that comes with being around/friends with them
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