|“||I have the face for it, don't I?||„|
|~ Kai's catchphrase|
Kai is the main antagonist of Kamen Rider Den-O. He is a Singularity Point from the future who is the mastermind behind the Imagin attacks, psychically giving them the orders to destroy the key to the Junction Point to ensure the future of the Imagin.
To pick ideal dates to send Imagin back to, Kai uses a black page-a-day calendar where he has several hosts' memories and his own to pick from. To find these dates, he can look into human minds for strong memories much like the Imagin. When his own memories are used, a past incarnation of Kai appears in the past only to dissolve into a pile of sand. Kai is also capable of opening up portals through time and travel through it without the use of a time-traveling train, and through this ability can also release blasts of energy capable of erasing everything but himself from time and space. After mentioning how he feels about an event, he says "I have the face for it, don't I?". He often has an emotionless or angry expression on his face when he says this. Kai also has psychic abilities akin to Ryutaros's, having the ability to control people as well as summon up a small troupe of break dancers. Despite his villainous methods, Kai believes that his goal of saving the Imagin's future is altruistic, having true feelings for their kind.
He originally believes that the key to the Junction Point is the elder Yuto Sakurai, traveling back in time to January 10, 2007, to kill him. However, when the changes to the timeline from Sakurai's death are undone, he believes that Den-O is the culprit, and sends the Imagin Ryutaros to the DenLiner with a permanent ticket with the charge of killing Ryotaro. When Ryutaros refuses to follow through with his mission, Kai reveals himself and tries to get Ryutaros to sabotage the ZeroLiner, taking the DenLiner and Den-O Belt out of commission. After this fails, he notices that Yuto does not protect his future self from harm in the past, but rather protects Ryotaro.
After questioning him, Kai discovers that Airi is the one being protected, and Kai sends an army of Imagin to kill her. As Airi's memory loss and other quirks confound him, Kai creates the Death Imagin from all his remaining memories and will. Kai's army destroys the city until he finally discovers that the key to the Junction Point is not Sakurai or Airi but rather their child, the Singularity Point Hana. After Den-O, Zeronos, and their Imagin partners protect Hana by destroying the Death Imagin, all of Kai's memories are used up and he dissolves away into sand, taking all of the Imagin except for the Tarōs, Deneb, Sieg, and a few other isolated cases with him.