I know a few things about MPD and have actually met a few people who have that diagnosis. It's much more disabling than mood swings, though.
And, yes, they are sort of spooky. But it must be a lot worse for them than others around them. I think the personalities are sort of like Johari's window. Some of them know what others others know and some of them don't.
One place where I worked we had a resident with MPD. She'd leave to go to work at the rehab center in the morning wearing a skit and blouse and come home in the evening wearing slacks and a tee shirt. Apparently she had another life and place we didn't know about. One of her personalities would steal things which she'd deny having and then she'd find the item in her coat pocket and be asking who's property it was. Sometimes she seemed to be having (very animated) conversations with one of the other personalities. Sometimes she seemed like a normal person, though not necessarily in her original personality. It was difficult for her to answer questions pertaining to what was going on with her. I think she spent a lot of time in confusion and had learned well to confabulate to cope with/hide it.
I saw her walking far from home one day and stopped to give her a ride. She wouldn't come near the car. She narrowed her eyes and asked, "Do I know you?" Since she was so vulnerable I was glad to see that whichever personality was in charge at the moment at least it was remembering the safety rules she'd been taught.
You'll get a much better description of the disorder from something like Wiki. There's probably a description of the different types of personalities which may manifest and their various functions and connections. Among them are a rascal, a child, an infant, an organizer, the person themselves, sometimes a person of the opposite sex. More. Pets? Not sure.
I know that a goal of therapy initially was to integrate the personalities and that sometimes this is not possible. It is suggested in some cases that the damage has been so severe that the original person has been totally obliterated. I think new theories of treatment have been developed.
The degree of damage is not necessarily the issue as much as the strength of the original personality's coping skills and circumstantial factors. What may shatter one individual may not do so to another.
Did you guys know that Sybil, from the story, came from near here and attended Mankato State? I think it was a normal school then. Teaching teachers. The teacher she had there was the psychologist who treated her. It was about ten years back that all this was finally revealed when the characters' privacy was apparently no longer an issue.