Kaiki Deishuu is a conman specializing in fake apparitions.

Kaiki has a gloomy personality and subscribes to the view that there is nothing in the world that is inherently true. The only absolute in life for him is money, due to its value in providing anything he wants that isn't irreplaceable; he even believes it can readily buy concepts like love and happiness. An adept schemer, Kaiki has an opaque mind to outsiders. He is fluent in the arts of lying and subterfuge, so the reasoning behind what he says is constantly blurred. He explains his name as "Kai," like a mound of clam shells, and "Ki," like a dead tree.

Kaiki is an individual who prides himself in having no pride. He openly admits to being a conman and an impostor, among other things, but often this is to his advantage as a master of wordplay. He flows easily in his interactions with others, and he is willing to give and take in order to maximize profit or cut his losses as needed.

Kaiki is a fake specialist who knows about paranormal creatures, but does not personally believe in them. Instead, he manipulates those who do believe, using his knowledge to make money off of them.

In "Nisemonogatari" he serves as the main antagonist of the "Karen Bee" arc. He is identified as the one who swindled the Senjougahara family, as well as the one distributing charms among junior students of Naoetsu High School throughout "Bakemonogatari" and "Nisemonogatari." Confronted by Karen Araragi for his actions, Kaiki uses a bee apparition to incapacitate her and leaves. He is confronted in the arc's finale by Araragi Koyomi and Senjougahara Hitagi, who demand he cure Karen's curse and leave town for good. Kaiki claims that the bee apparition was a fake, a simple trick of hypnosis, and that whatever symptoms she is suffering will pass in a matter of days. He then agrees to their demands and decides to leave the town for good, though not before some choice words to Senjougahara. It is later revealed in the "Tsukihi Phoenix" arc that he was a colleague of Kagenui Yozuru and Oshino Meme during their days in university, having supplied the former information regarding Araragi Tsukihi.
In "Monogatari Series: Second Season," he serves as the narrator and main protagonist in "Koimonogatari." He is hired in secret by Senjougahara to deceive the human-turned-snake god, Sengoku Nadeko, who had vowed to kill Araragi and his loved ones. Despite being prompted by Gaen Izuko to back out, Kaiki persists and narrowly succeeds in convincing Nadeko to give up her godhood and attachment to Araragi by appealing to her (well-hidden) former dream of becoming a mangaka. During the arc's resolution, just as he is about to leave town he is attacked by one of the junior high students that he scammed in "Nisemonogatari," evidently tipped off by Oshino Ougi and implied to be the same student that had previously cursed Nadeko with a snake apparition.
Having survived the attack, Kaiki meets with Kanbaru Suruga during the events of "Hanamonogatari," where he reveals that the "woman of his dreams" was Kanbaru's mother, Gaen Tooe. Gaen entrusted Kaiki with Suruga's well-being before she died; in honor of this promise, Kanbaru is the only one whom Kaiki shows unconditional kindness and never attempts to swindle. He offers Kanbaru his card—with the usual "ghost buster" moniker scratched off—and tells her to call him if she's ever in need. Not long afterward he sends her the mummified devil's head, which he claims was given to him by Gaen.