Kevin is a strange human Monster kept in the Facility. He is listed on the Whiteboard but is not bet on. It is unknown if he is seen during the System Purge.
Physical Description and Attributes
Kevin is a normal-looking adult male. He has no striking or outwardly features that distinguish him among the other Monsters, making him blend in with any other ordinary looking people. This is extremely dangerous as he can use this deception to lure away and convince his victims he is not a threat, only to kill them later. Since he is only mentioned in the novelization of the film and never seen on camera, his physical description remains unknown. Kevin could be your average serial killer, but that wouldn't be enough reasoning for the Facility to capture him. It's possible he could be suffering from a severe mental illness that makes him extremely uncontrollable and unpredictable. Or, Kevin could be the host for some demonic entity, doing its evil deeds through Kevin's body, however this is just speculation and not a confirmed fact.
In the Cellar, he is almost summoned by Marty by tampering with film tape, which is associated with Kevin.
He is not seen during the film.
In the novelization, Kevin is described as a quiet, normal-looking person with a small smile on his face who calmly walks through the post-Purge chaos until he comes across an injured guard. He then proceeds to "exsanguinate him in a second." When Dana finds the guard, he appears "untarnished, but Dana had never seen 'anyone more dead.'"
Due to his unremarkable appearance, any normal looking human seen in the Purge could be Kevin.
The fact that he is identified only by first name and never seen or referred to otherwise indicates that Kevin may be a trope based upon the concept of a person with an outwardly normal appearance and nonthreatening demeanor who is nonetheless extremely dangerous.
Kevin is a reference to Sin City's Kevin, who fits the same description. A popular Canadian cartoon called Kevin Spencer also exists. The show takes place around Kevin and his alcoholic, sociopath antics.
Director's comments reveal it is most like the title character "Kevin" from the novel and film We Need to Talk About Kevin, in which the child Kevin displays sadistic violent tendencies.
The fact that Kevin is summoned by a film strip may also be a reference to the character Patrick Bateman, from American Psycho. Bateman uses the excuse, "I have to return some videotapes", to leave a social situation and follow up with going on a murder spree.
It is possible he was inspired by the Juggernaut from Thir13en Ghosts, as both were serial killers who would dismember people.
Other examples include Norman Bates from Psycho or any number of real-world killers who did not around suspicion to their acquaintances as they committed their crimes, such as Chilean serial killer Simon Busques.