Forrest Kaysen is a recurring character in titles directed by Hidetaka Suehiro, a.k.a. Swery. Kaysen's role differs depending on the game he appears in, and his appearances are not confirmed to link with each other in any way. In each of the games he has appeared in thus far, he bears the appearance of an overweight middle-aged man with short hair and glasses.
Kaysen's first appearance is as a scientist, Dr. Forrest Kaysen, in Spy Fiction. He is a supporting character who is killed partway through the story.
The Kaysen of Deadly Premonition is a traveling tree sapling salesman with a lighthearted personality. He is later revealed to actually be a demonic agent of the Red Tree and the game's main antagonist.
The Clock Tower
In Deadly Premonition, Harry Stewart tells York about his encounter with the original Raincoat Killer. He climbs the town's clock tower to find the military releasing purple gas out into the air that causes the town's people to go into a rage and attack each other. Forrest Kaysen is there, the mastermind behind the whole operation.
The Red Seeds
It is revealed that Kaysen is some kind of immortal demonic entity who enjoys growing plants that require the sacrifice of a human life to bloom. In a flashback, York (as Zach) sees that Kaysen was the one who killed his mother by using the red seeds to grow a tree in her stomach. His father wanted to kill her to end her misery, but he instead turned the gun on himself. After this scene Zach breaks out of the white room and goes back to the real world. Now Zach has to deal with a similar situation to which his father was in. Emily Wyatt, the girl that he loves, was also abducted by Kaysen and has a tree grown out of her stomach. Zach's father tells him to not make the same mistake as he did, but in the end he is unable to shoot Emily to put her out of her misery. Instead Emily kills herself by pulling the red tree out of her stomach. After having some final words with Emily, Zach battles Kaysen, who is surprised that Zach broke out of the white room. In the end Zach defeats Kaysen, and Emily "lives" happily ever after with York by her side.
In the episodic mystery D4, Kaysen is a police detective with a gluttonous appetite and partner of the game's protagonist, David Young.