Dissociative Identity Disorder used to be called Multiple Personality Disorder, but it was renamed to focus on the symptoms that were actually harmful, because being plural isn't a sickness. Plurality can also arise from other disorders such as Narcissistic Personality Disorder, and it can also arise naturally or as the result of deliberate or accidental efforts. It's often linked with spiritual beliefs by people who experience it in a way which conflicts with normative reality's narratives. Leonard Nimoy famously has a Spock in his head who he sometimes talks with, because of all the years he spent playing Spock on TV. The brain adapts.