**There are major spoilers 
for this novel in the discussion below.**
Think about it like this. You refer to yourself as "I" or "me" because even though you act differently at work or school than you do at home, you experience yourself as a single whole. According to the original "multiple personality" view, the person housed multiple selves or personalities in one body, the same way someone who is "possessed" is viewed as holding more than one conscious entity within one skin.
Now theorists believe there's only one self or personality, it's just been splintered by trauma. The purpose of the splintering is to protect the birth or core personality from the terrible things that happened. It's the same process that happens when any person finds an experience too painful to remember, as after a car accident or some other trauma. The amnesia is a defense mechanism to keep the person from having to think about unbearable psychological pain.
So all of the alters are part of a single whole, and all of them were created out of the same person (ie the "core"), and although the birth personality doesn't know about them, they may know about the birth personality and about each other.
Some people believe that DID is nothing more than a hypnotic state, partly because it's so easy to temporarily "create" DID, it can be done in a classroom in front of a group of students! (Obviously, I hope, there is no trauma involved.)
Hypnosis is another word for dissociation, and all of us dissociate. Dissociation is an altered state of consciousness—it’s what happens when you’re driving and you suddenly realize you don’t remember the drive because you were thinking about something else. It’s what happens when you’ve watched a really great movie that felt like it was an hour long when it was actually two. It’s what happens when we’re daydreaming and we have to ask someone else to repeat what they said.
Part of the argument against DID is that all of us have different parts to our personalities. How we behave in our bedrooms is different than how we behave in our workplaces; how we behave in front of our families and in-laws is different than how we behave when we go out for a night on the town with friends.
For those who believe in DID, the belief is that the amnestic barriers between the dissociative states are experienced as stronger than usual, making each state feel like a different personality. So the part that gets angry in the grocery store experiences itself as separate from the part that behaves politely at Thanksgiving dinner.
Most of us can move fluidly among our different parts; in DID, since the different alters developed to deal with different environmental demands, untreated people with DID jump from one state to another based on what's happening around them.
One of the key symptoms of DID is missing time. Because the alter that’s “out” is the one making memories, the rest aren’t. In an old video of the real "Eve" (from the Three Faces of Eve), the interviewer asks one of the personalities where she went when the others were out. She thought about it and then said, "I don't know. Somewhere." She kind of likened it to sleeping through class. She was there, she just wasn't bothering to pay attention.
So if the core personality is only “out” in the evenings, it will have no memory of going to school. This is how different alters can sometimes speak different languages. One alter was paying attention in German class and the rest weren't!
The gaps in memory are so noticeable that they’re often what bring people with DID into therapy. The people realize when they reach college or get their first job or whatever that other people don’t have these enormous blank spaces.
