It is mentioned in the film that Kevin's father left on a train and presumably died (possibly in the Eastrail 177 crash in 2000 caused by Elijah Price). Kevin was primarily raised by his extremely abusive mother who suffered from obsessive compulsive disorder and threatened her son with a clothes hanger. One of the earliest personalities to form was that of "Dennis", who came "into the light" when Kevin was 3 years old and cleaned everything in order to keep his mother happy and unabusive; "Dennis", who in Kevin's adulthood and by proxy his as well, also has OCD as a result.
For about ten years before the events of Split, Kevin worked at the Philadelphia Zoo, eventually becoming head of maintenance, and his DID was treated by Dr. Karen Fletcher. The real Kevin was buried in his own mind by his other personalities on September 18, 2014 while he was on a bus.
Kevin Wendell Crumb - The host body for all of the personalities.
Dennis - He is one of the earliest personalities to form, as well as one of the more disturbing, as he has a perverted liking for watching young girls dance naked; he also has obsessive compulsive disorder and touches everything with a yellow rag. He is also very intelligent, manipulative, and tough, and is mostly able to act like a different personality. Although he is cold at times, he seems to have a lighter side especially towards children. He, Patricia, and Hedwig are known as "The Horde" by the other 20 personalities, and the three are worshippers of The Beast.
Patricia - A polite British speaking woman who at times can be psychotic. She is very knowledgeable when it comes to animals and cuisine in different societies, and loves to cook. Dennis and Hedwig listen to her and all three are called "The Horde".
Hedwig - A 9 year old boy who befriends Casey, Claire, and Marcia after they were kidnapped by Dennis. He loves to draw and dance to music, especially that of Kanye West. Unlike the other personalities, Hedwig has the power to steal "the light" from Barry, the dominant personality, and while not intentionally evil, Hedwig chooses to give the light to Dennis and Patricia so he's not made fun of.
Barry - An extroverted, effeminate fashion designer who is one of the nicer personalities, and is the dominant personality before being taken over by The Horde.
The Beast - The 24th and most dangerous personality to form. This personality changes Kevin's body chemistry and physiology to the point where his skin is impenetrable, and can develop superhuman speed, strength, and durability.
Jade - A foul mouthed yet sweet teenage girl who has diabetes and has to take insulin shots.
Orwell - A highly intelligent man in the field of history who tends to compare historical events, mostly those in the 11th and 12th centuries, to the feud between The Horde and the other 20 personalities including himself.
The other following personalities names appear on a video diary log, even though they are not seen in the movie:
Heinrich - Also mentioned by Dr. Fletcher
Samuel - Also mentioned by Dr. Fletcher