Forrest Kaysen is the main antagonist of the video game, Deadly Premonition. He is a twisted extra-dimensional being from the "Red Tree Dimension" and the true mastermind of all Red Seed Murders.
Introduced as a jolly traveling salesman, who crosses the country selling his red saplings, Kayson initially seems completely uninvolved in the Greenvale Murders. He is at one point found by the protagonist, FBI Agent Francis York Morgan, in the house that a murder had taken place in minutes before. However, he is found locked up under the floor and is able to wave off suspicion by saying he and the victim were meeting to have a clandestine affair when someone came and interrupted them, causing her to hide him.
He later assists local cop Emily, York's love interest, in finding him when he is kidnapped. This cause York to trust him enough to have him drive Emily to the hospital after she is stabbed later in the game, while he goes to confront the Raincoat Killer. It is when he arrives and she is not there that York realized that he had missed something.He confronts Kayson theater, where he finds Emily with a red Sapling growing from her stomach. It is then explained that York's imaginary friend Zack is actually his real personality, and that Kayson did this exact thing to young Zack's mother. She begged her husband to end her life before it spouted, but he could not. The Sapling sprouted, and she died a horrible, agonizing death. Seeing this caused Zack's father to take his life in a fit of despair. Kayson, who had been off to the side laughing at the whole affair, found the hiding Zack and clawed him, giving him his distinctive scar and the creation of the York personality. This realization releases Zach from the red room. Kaysen becomes angry that Zach has escaped and proceeds to attack him, revealing his true form as Lovecraftian Horror. After a long battle, Zach is finally able to destroy Kaysen by shot his true body (which resembles three fleshy tentacles with the middle, tentacle holding the big red eye) and stop the spreading of the red seeds.