If you've seen Fight Club or Split you get the idea.
Severe childhood trauma, often sexual, can cause disassociation, paranoia, delusions of grandeur, and fantasies. Sometimes this side effect soup manifests as alternative identities the victim uses to armor themselves. World-renowned neuroscientists still lob competing theories of consciousness at each other, so it shouldn't surprise people that a disorder involving multiple identities is contested by a significant portion of the psychological community.
Billy Milligan is the most famous case of MPD/DID for a few reasons. He has the most alters by a mile (24 distinct personalities split between the core 10 and the 14 banished "undesirables") but also set a legal precedent as the first violent criminal (serial rapist) to be found not guilty by reason of insanity.
When Billy needs a skill one of his alters possesses, he's put to sleep so an alter can "take the spot." A group of alters put him into a 6 year coma due to repeated suicide attempts.
Philip is a petty criminal. Kevin, a drug dealer. Adalana, the affection-starved lesbian who “used” Billy's body in the rapes. David, the 8-year-old “keeper of pain”. Reagan, the Yugoslavian keeper of hate and expert marksman. Arthur, the English intellectual with a love for hematology. Allen, the electronics expert. Tommy, the escape artist. And all the others, including women, several children, a gorilla, and the Teacher, the only one who can put them all together. Reagan claims control over his adrenaline levels and has been witnessed by many performing inhuman feats of strength when he's on the spot. They've witnessed Tommy, who claims to be able to manipulate the muscles in his hands, slip out of handcuffs and straight jackets. He's sold portraits for thousands of dollars as a nobody in the '70s.
The biography takes witness accounts, recording transcripts and interviews between Billy and Daniel to recreate Billy's movements throughout the '70s up until publication in '81. A sequel entitled "The Milligan Wars" was published in '96 in Japan but was banned from US publication due to a suit by Billy against one of his hospitals.