The Cabin in the Woods' ending features almost every monster the film had teased to that point, except for someone mysteriously known as Kevin. For a horror fan, The Cabin in the Woods basically amounted to a love letter. Tribute was paid to the tropes of just about every horror sub-genre, from slashers to ghosts to giant monsters, and beyond. It's the kind of film that dedicated horror devotees will watch and grin from ear to ear, appreciating all the sly in-references to beloved classics.
During The Cabin in the Woods, the staff working to facilitate the various scenarios leading to the sacrifices needed to avert the apocalypse is seen placing bets on which monsters will end up coming into play. These are listed on a large whiteboard, and include such off-the-wall options as a unicorn and a merman. Thankfully, just about every creature on that list participates in the delightful "system purge" sequence that comprises most of the last act.
The big exception is Kevin, who's listed on the board, but doesn't appear later. He does, however, turn up in a deleted scene included in the movie's novelization, and there are various theories circulating among fans as to what movie he might be a reference to.
The Cabin In The Woods: Who Is Kevin?
According to The Cabin in the Woods novelization, Kevin is exactly what his name might imply. Instead of being an obvious monster, he's instead a normal looking guy that no one would likely ever suspect of being dangerous. During the purge sequence in the book, Kevin casually strolls through the massacre taking place all around him, smiling the whole way, before calmly kneeling down next to an injured guard and exsanguinating him within seconds. Due to these traits, it's possible Kevin is the deadliest foe of all, as had the characters in the film encountered him, they might've mistaken him for another survivor, only to realize their mistake when he began murdering them.
As far as what film Kevin might be a callback to, one of the most popular choices is Elijah Wood's character from Sin City, who fits Kevin's description quite well. There's also Norman Bates from Psycho, a decidedly mild-mannered fellow hiding a psychopathic dark side underneath. One of the most likely inspirations appears to be the titular character from We Need to Talk About Kevin, about a psychotic teenager who ends up going on a murderous rampage. While the film adaptation starring Ezra Miller as Kevin didn't come out until after The Cabin in the Woods had been shot, the book it was based on released in 2003.