I also thought of multiple personality disorder before I posted my description, hah. Interesting to see you pick up on it right away.
I liked Aylen's take on it. I'd add that even if a personality is fragmented, that fact might point at a certain essence or core being.
It is easier to imagine the core to be whole, but it might be fragmented or discolored or torn apart, and still count as the essence of something.
I am not an expert on the subject, but with MPD, it seems like the individual has compartmentalized certain experiences and mindsets due to trauma, and keeps replaying them to cope with it. Now what the essence of that person is exactly, is not easy to tell and depends on the individual.
We may strive for clear answers, but the journey of finding them matters to Ni as well. Ni is not necessarily always concerned with the definite, though that is its aim. Sometimes the journey has more value than reaching the destination itself.