Kevin Wendell Crumb is a man who suffers from an extreme case of Dissociative Identity Disorder, and has twenty three separate personalities residing in his mind, even though Kevin means no evil, his other personalities are each one rather vicious in their own way, however his twenty fourth personality "The Beast", a result of Kevin job in the zoo and a combination of the animals there is the film's true main antagonist, and is revealed near the end of the film, his other two most evil personalities and believers of "The Beast" are Dennis an obsesive, cold and tough man, who acts as the film's main antagonist for most of the film, but when "The Beast" existence is confirmed he turns into the secondary antagonist and Patricia a female persona, sweet most of the time but secretly evil, she acts as the films tertiary antagonist.
When Kevin was a child, his father abandoned him on a train, leaving him to live with his abusive, obsessive-compulsive mother. As he was abused, he began to develop multiple personalities as a defense mechanism, ultimately developing twenty-three individual personalities, each of which spontaneously took control over his body and subjugated the original Kevin. When he grew up, he worked at the Philadelphia Zoo, and one day, two girls approached him and played a prank on him, taking his hands and putting them under the girls' shirts, on their breasts, before running away giggling. This left Kevin confused and vengeful, and possibly contributed to some of the appetites of 'Dennis'.
At the beginning of the film, Kevin attacks Casey, Claire and Marcia, knocking out Claire's father and abducting the three girls. He takes them to his home, where he confines them to a room, where he perplexes and frightens them with the demonstration of his several personalities - however, when he becomes Hedwig (A 9-year-old boy with a lisp), Casey tries to befriend him. Throughout the film, mainly through the discussions between Kevin and his shrink Dr. Fletcher, it is established that the dominant personality is Barry, who tries to subjugate the others and keep them from staying 'in the light', however two of the other personalities (Patricia, a woman, and Dennis, an Obsessive-Compulsive man with a sexual appetite) begin to combat with Barry for dominance.
When Claire tries to escape the room through a fresh drywall, 'Dennis' corners her and then locks her in a separate room on her own, and when 'Patricia' dines with Casey and Marcia he forbids Claire from attending them for what she has done. When Marcia attacks him and tries to make her own escape, he outpaces her and locks her also in a separate room, dividing all three girls from one another and worsening the trauma they experience at his hands. Casey, resourceful and used to being abused by an adult, befriends 'Hedwig', and tries to influence him. Hedwig excitedly warns the girls about 'the Beast', a malevolent 24th personality with incredible abilities. He also reveals to Casey that there is a window in his room - she perceives it as a potential escape route. However, when he shows her the 'window', it is shown to be a hand-drawn picture of a window.
She grabs a functional walkie-talkie nearby and radios for help before being subdued by 'Dennis'. Barry sends several email messages for help to Dr. Fletcher, revealed to be Barry feeling threatened by the bid for control by Dennis and Patricia. When Dr. Fletcher approaches and tries to reason with Kevin, she realises that he has kidnapped the girls and tries to convince him that what he is doing is wrong. 'Dennis' drugs her and locks her up. Enacting a sort of ritual, Kevin then buys flowers and visits a train car, before becoming 'the Beast'. Dissolving into an animalistic mentality, Kevin goes on a rampage. When she recovers, Dr. Fletcher writes down Kevin's full name, knowing that it will be the singular way of subduing him once he transforms into the Beast. The girls, in the meantime, try to escape from their confinements.
Returning to his hideout, 'the Beast' confronts Dr. Fletcher. She tries stabbing him with a knife, but the knife shatters. The Beast then murders her, demonstrating incredible physical strength. He then proceeds to savagely kill Marcia and then Claire, while Casey stumbles upon Dr. Fletcher's corpse as well as her instructional note. When the Beast closes in on her, demonstrating the ability to climb up walls, Casey uses Kevin's real name to call him to the light. The Beast is briefly tamed, and the actual Kevin (who has been restrained for a very long time by the other personalities, and has no memory of what he has done between then and now) is shown to be horrified by what he has done, and orders her to kill him with a shotgun before the Beast can return and defend itself - however, the Beast is alerted when Kevin's other personalities desperately try to defend themselves. As the Beast reawakens, it engages in a climactic and violent showdown with Casey. Casey fights the Beast, shooting him several times with a shotgun, but it proves to be superhumanly tolerant to pain.
At the climax of the fight, the Beast voices his maniacal plans to rid the world of the 'untouched' (As in, the people who he feels have never suffered pain or discomfort, unlike him). Casey is cornered in a cage, but the Beast bends the bars with its bare hands and is about to attack her when it notices the scars on her body from her previous abuse at the hands of her uncle - it rejoices in the fact that she is 'pure' (she has suffered). Speculating that troubled people are exceptional, the Beast decides to spare Casey and flees, while Casey is eventually discovered and rescued by Kevin's co-workers at the Philadelphia Zoo. Recovering from the battle, Patricia, Dennis and Hedwig speculate in awe at the power of the Beast, marvelling at what it was shown to be capable of doing. Kevin voices his plans to change the world, and awaits the next feeding of 'the Beast' now under the man's newest and diabolical collective nom de guerre "The Horde".
- Kevin himself: A terrified young man who, from what can be seen at the end of the film, has a conscience and expresses deep shock and horror at what his other identities have done. He is so horrified by what he has done that, after realising the destruction he has caused and the trauma he inflicted on Casey, he encourages her to kill him, and the only thing that prevents this from happening is the frantic interference of the other personalities that existed in his head. He spends the majority of the film restrained by these personalities.
- Hedwig: A nine-year-old boy with several compulsive behaviors, such as proclaiming that he has red socks, crawling on bent knees, and ending sentences with 'et cetera'. He loves to dance crazily, do drawings and he is disciplined by the other personalities. He befriends Casey.
- Barry: The originally dominant personality in Kevin's head - calm, level-headed to some extent and working to control the other personalities, Barry is the personality that is used to communicate with Dr. Fletcher, but he is exploited by Dennis in the latter's bid for control over Kevin's head. He's a charismatic, somewhat effeminate male with an incredible talent for fashion and design.
- Dennis: One of the more disturbing personalities, Dennis is temperamental and has a perverted liking for watching young girls dance naked. He demonstrates traits of obsessive-compulsive disorder and a firm tendency towards cleanliness and order (which is possibly duplicated from Kevin's abusive mother). Dennis is devious, being shown to have been masquerading as Barry since the very beginning of the film.
- Patricia: A sophisticated, orderly, polite 'woman' who has considerable command over some of the other personalities (This personality claims that the others 'listen' to her). Patricia is very disciplined towards the other personalities, and towards other people, and is also very calm - she assures Casey, Claire and Marcia that Dennis will not touch her because they are to serve a greater purpose than Dennis's desires.
- The Beast: By far the most dangerous, hostile and terrifying of all the personalities, the Beast is the 24th identity that resides in Kevin's mind. The Beast is a malevolent figure, idolized by Dennis, Patricia and Hedwig, who all await his arrival. The Beast is extremely violent to the point of savagery, having an unquenchable lust for human flesh, and an intense perspective that the rest of the world are impure because they haven't suffered like he has. The Beast possesses superhuman strength, speed, agility, stamina, invulnerability and pain tolerance - it can rip metal bars apart and snap a human body in half with its bare hands. It also has the ability to climb walls using even the slightest grip. Finally, the Beast is incredibly hard to kill, since bullets are virtually ineffective against it and knives shatter against its skin. The Beast is viewed as an almost Godlike figure by the other 23 identities, and they are in awe of what it can accomplish.
- Jade: She appeared on a video log and briefly appeared after Casey called Kevin into the light. She apparently has diabetes and needs to take insulin shots for it.
- Orwell: He appears briefly when Casey calls Kevin into the light. Orwell is an introverted, highly intelligent man whose verbosity tends to overshadow his intended meaning.
There are 16 other personalities that are not shown, but the names of which are seen on Kevin's computer:
- Mary Reynolds
- Mr. Pritchard
- Casey Cooke - Prisoner
- Dr. Karen Fletcher - Psychologist turned Victim
- Claire Benoit - Prisoner turned Victim
- Marcia - Prisoner turned Victim
- Mr. Benoit - Victim
- David Dunn