Separation of Psyches

From Zanecorpwiki

This is the idea that one of the big challenges author's face is a separation between their characters. Many books, in my opinion, tend to have characters who are all of a particular shade. Many more have a simple two-side-to-a-coin approach. The books that have a range of characters who each feel like fully fleshed people with different backgrounds and separate minds are rare.

Implicit Convergence is a related topic.