So, disclaimer, my formal education on psychology is limited to taking Psychology 151 as one of my social science requirements at community college, but I've been reading a psychologist's blog since beforeTrump was elected and I've read a number of stories in the rational fiction genre, where effort is generally made to get character psychology right instead of just having characters act as the plot demands. Also, my apologies if anything I say below comes across as insensitive as I am not all that fond of the euphemism treadmill, extreme political correctness, or wokism and find clear communication hard enough without constantly worrying if I'm going to accidentally offend someone.
I'm not really bothered by this subject, though it took me a while to figure out both OP and their Girlfriend are plural as my prior was that plurality is a trait rare enough it is unlikely one person with the trait would meet another person with the trait organically. Granted, among the rational fiction I've read include Harry Potter and the Methods of Rationality and Pokemon: The Origin of species, both of which feature viewpoint characters that, if not plural in the same sense as OP and their Girlfriend have at least some superficial similarities and those aren't the only things I've read with such characters, so even if I'm not fully understanding what's happening here, it doesn't trigger feelings of alienation. Though, at the risk of asking a possibly offensively ignorant question, is plurality in this sense remotely related to the concepts of Internal Family Systems or Tulpas.
I don't believe it's possible to copy a personality from one mind to another the way one would a mp3 file from one's harddrive to an SD card, but it is certainly possible for one mind to model another or for someone to subconsciously imitate the outward facing traits of someone they admire and become more like them. Add plurality to the mix, and it seems feasible that one's mental model of another person could ascend to being a full alter or that a newly created alter, unbound by habits of its headmates, could more efficiently follow the path of mimicry. The result would be less like copying a file and more like recording an Orchestral performance with a single, monaural microphone. Granted, this is entirely conjecture, and I'm assuming OP and their girlfriend have known each other long enough for the girlfriend to know OP well enough to have a good mental model of OP.
Speaking more generally, I'd recommend to anyone that they seek mental health services on a regular basis, the OP and everyone else in this thread included. Not because I think plurality in and of itself is a red flag for serious mental illness, but because, as one of my favorite science communicators is fond of saying, "Mental health is health", and in an ideal world, an annual mental health evaluation would be as routine as an annual physical, semi-annual tooth cleaning/dental exam, and annaul eye exam, and the recognition that finding health problems early is beneficial both to the individual and to society as a whole, there would be social programs to ensure even the poorest person can recieve such baseline care... and even mentally healthy people sometimes need an impartial third party to bounce ideas off of to help them work through tangled thoughts, deal with unfamiliar emotions, or cope with unexpected life events.
Also, I hate the terms neurotypical and neurodivergent and feel like the DSM and other reference documents overuse the terms disorder and syndrome. There is no such thing as a typical mind, just a massively multi-dimensional space of possible minds, most of those dimensions probably following a normal/binomial distribution and with the vast majority of minds being at least one standard deviation from the mean on at least one dimension(the chances of being within 1 SD of the mean across 5 independent variables is less than 15% and across 10 independent variables is less than 2%), and while it's useful to have bundles of traits with labels that make talking about them easier, considering how often those labels have syndrome or disorder in them, it feeds into the baseline assumption that having any of these specified bundles of traits automatically means one has something seriously wrong with them.
And @40: I can't rule out latent plurality, but I think some of that might just come down to the fact most people have at least a few inconsistant or downright contradictory thoughts, and that most people, even really smart people, tend to vary wildly on how smart they are when it comes to specific subjects(I've transplanted SATA harddrives from one computer to another several times, but I have absolutely no clue how to change the oil in a car).