I once encountered the opinion that if one headmate has a disorder, then the whole system must have it because that means your brain has it.
I don't know how to express this, but, that isn't true for every system.
I haven't been diagnosed with NPD despite meeting the criteria, so I won't claim to have it. However, I definitely have some sort of pathology going on. Yet, my system members do not. I see myself as special and superior while being actively disgusted by and easily insulted by others. I will crash out over stupid little perceived slights. I must always be the best and get constant attention or else I become extremely disregulated. I also see people in terms of how I can use them. I find it amusing and gratifying to manipulate people (I avoid doing it with harmful intentions as a matter of principle) and satisfying to utterly destroy them (when I decide they deserve it). I have very, very little empathy for people that I don't personally have some sort of affection for, or at the very least, that I find useful.
Meanwhile, we have Max, who's a genuine sweetheart. He loves helping people. He's full of empathy, even for people who have mistreated him before. He's very understanding and is perfectly happy to listen to and comfort people in distress. He sees others as potential friends and likes to give people opportunities to be better. He's sensitive, sure, but he doesn't have outbursts over it. He's much quicker to go right back to playing and being positive. A literal puppy dog. I don't think he could manipulate or use someone if he tried.
Even Alex, who thinks of himself as the sexiest man in the world, isn't bothered by people disagreeing or even insulting him. He laughs it off and just considers it their loss. He's also extremely laid back and isn't really upset over people having different opinions (save for bigotry ect) from him. He rarely snaps at anyone unless it's in defense of his loved ones. He's not the best at having emotional conversations, but he's not disgusted by people expressing emotions. He will offer comfort to the best of his ability and find fun distractions to focus on. He's more level headed and can tell more or less when something is a big deal or not worth reacting over. He stands up for others when he sees someone being mistreated. He has a good sense of self and he's proud of who he is most of the time. He doesn't feel the need to see others as inferior or to use anyone because he's already secure. He's competitive but not to the point of being completely dejected if he isn't the best.
We also suspect I have autism as it's been suggested by our therapist. Max fits the criteria for that as well. Both of us are extremely easily overwhelmed, we have various needs for stimming, we're sensitive to various outside stimuli, we have meltdowns, and we struggle to understand certain social cues or implications. I'm much better at masking all of that than Max, but I digress. I'm also excessively meticulous and detail oriented.
Alex on the other hand, isn't concerned with little details. He's capable of handling most tasks (save for ones that require a bit more of a thought process) that would overwhelm us two. He's not nearly as sensitive to stimuli that would otherwise overwhelm Max and I. He's less socially awkward with no anxiety. Zero problems understanding that words aren't often specific or literal. He doesn't feel the need to stim as much. He doesn't really melt down unless it's excessive bleed over from me, or unless I personally push him to do literally everything for us for months on end. If he is autistic, he's very low support needs compared to us other two.
The only thing we really all seem to display symptoms of is ADHD because we all struggle with focus, memory, rushing around with tunnel vision, and getting overly excited or distracted by something.
Not sure if I did a good job detailing my points, but I wanted to get the idea out. My point being, if your brain does function the same for your entire system, that makes sense and is valid. It also makes just as much sense if it doesn't because you are all different individual people whether you share a brain or not.