For info, Split and movies like it have done near irreparable harm to the system community. It's because of the reinforcement from media like that that the stigma against systems still exists and is so hard to get rid of.
For example I know of a system that after their university found out about their diagnosis refused to let them back into university despite them meeting all the conditions initially set by the university after they had a breakdown from stress. The university's reason was that 'they would be a danger to other students'.
It's very common for some of the first questions people ask when they find out someone is a system is "who is the 'evil' one?" "am I safe around you?" or "do you have a monster in there?". Studies have found that systems are far more likely to be a danger to themselves because of the trauma they have suffered than anyone else.
I know you probably never thought of this, but it's good to keep in mind that you compare or are reminded of a horror movie when thinking about trauma survivors with DID and maybe you might want to consider the effect that might have on the way you might treat IRL systems ^.^
I understand that there's a big stigma regarding mental health, and it's always treated or seen as something negative and linked to something "bad" but society is always afraid of the unknown or different and doesn't want to do something about it either and think that these people are social outcasts and
"split" particularly don't help much to change that stigma.
This manga reminded me of that movie because of two personalities (the kid and the girl) and just wanted to mentioned it... I apologize if it was somehow rude or offensive :(

This reminded me of the movie "Split"
Also, i love this mangakas drawing style and the stories never disappoint