Deishuu Kaiki (貝木 泥舟, Kaiki Deishū) is a conman specializing in fake apparitions.
Kaiki has a very 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 wishes that isn't readily irreplaceable. A schemer at heart, Kaiki has an opaque mind to outsiders. He is fluent in the arts of lying and subterfuge, so his 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 imposter amongst other things. But this is to his advantage as someone who deceives with wordplay extremely well. His interactions with others are very fluid, and he is willing to give and take in order to maximize the outcome, or cut his losses, in his favor.
Kaiki is a fake specialist who knows about paranormal creatures, but does not believe in them, so he prefers working as a conman by manipulating those who believe in apparitions or just using his knowledge to make money by fooling people.
In "Nisemonogatari", he serves as the main antagonist of the "Karen Bee" arc. He is revealed to have swindled the Senjougahara family and has been the cause of the many scams and distribution of charms among junior high students of the Naoetsu school since the events of "Bakemonogatari" up to "Nisemonogatari". Confronted by Karen Araragi for his actions, Kaiki uses a bee apparition to halt her from attempting to stop his business. He is confronted in the arc's finale by Koyomi Araragi and Hitagi Senjougahara, threatening him to cure Karen's curse and to leave the town. Kaiki reveals that the bee apparition was merely a fake and that he used nothing more than simple hypnosis. He promptly agrees with the two and decides to leave the town for good. It is later revealed in the "Tsukihi Phoenix" arc that he was colleagues with Yozuru Kagenui and Oshino Meme during their days in university, having supplied Kagenui information surrounding Tsukihi Araragi.
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, Nadeko Sengoku, to halt her desire and promise to murder Araragi and everyone attached to him. Despite being prompted by Gaen Izuko to pull out, Kaiki persists in his job and succeeds in convincing Nadeko to give up her godhood and attachment to Araragi, prompting her to pursue her true dreams. During the arc's resolution, just as he is about to leave town he gets beaten by one of the junior high students that he scammed in "Bakemonogatari", seemingly tipped by Ougi Oshino and implied to be the same student that had previously cursed Nadeko with a snake apparition.