Daiki Kaito (海東 大樹, Kaitō Daiki?, portrayed by Kimito Totani) is a mysterious young man who is portrayed as being cheerful to the extent of being annoying to those around him, but becomes serious in battle and sees material objects as more important than anything else. Narutaki, aware that Daiki is Diend, avoids getting in his way. He also appears to know about Tsukasa's past, referring to sea cucumbers as being something Tsukasa had always wanted to taste. He travels from A.R. World to A.R. World, stealing what he refers to as "treasures". Daiki's surname is a homophone of the Japanese word kaitō (怪盗?, phantom thief), a role he fulfills in Decade. In some materials in Decade, Daiki's surname is spelled in English as "Kaitoh" rather than "Kaito".
It is revealed that he comes from an A.R. World (referred to as the "World of Diend") where he was an officer who served the evil Fourteen who has deemed Kamen Riders as threats to the public peace. Daiki hunted them down and turned them in for education until he captured the leader of the Kamen Riders: Kamen Rider Glaive. When Glaive is unmasked, he is revealed to be Daiki's older brother Junichi. By then, Daiki learns the truth that Fourteen brainwashes people, powerless to stop him. Losing his self-confidence and seeking to undo all of the mistakes he had caused, Daiki stole the Diendriver from Dai-Shocker, becoming Kamen Rider Diend and Fourteen's new most wanted Kamen Rider. He leaves his world, traveling to other worlds and taking their treasures as a means of self-delusion until he crosses paths with Kamen Rider Decade.
Eventually, he does lackadaisically accept the Hikari Studio residents as "friends", even calling Decade a "comrade", though he is not beyond abandoning them without explanation, usually for his own self-interest. Eventually, he returns to his world, and along with Decade, finally frees it from Fourteen's tyrannical "peace", and settles matters with his brother. Afterwards, he plays a pivotal role in the defeat of Dai-Shocker, providing Decade with the Final Attack Ride Diend Card which allows the two to finally destroy Super Apollo Geist, as well as using the Final Form Ride Decade Card to allow Decade and Kamen Rider J to destroy King Dark. Following the Rider War and Decade's death, he assists in reviving Decade, first by holding off the Super Shocker army so a photograph of Tsukasa can be recovered, then by using his memories to help bring him back to life. Afterwards, he takes part in the final battle with Super Shocker and, following Ultimate D's demise at the hands of Decade and Double, returns to the Hikari Studio to continue his adventures with the group.
Returning to the World of Den-O during the events of Episode Yellow: Treasure de End Pirates, Daiki allows himself to become possessed by a Spider Imagin with a contract to meet Ryotaro. This is part of a scheme to travel back in time to November 22, 2008, and steal a gold-plated gun from the Kurosaki family estate, a heist he failed to perform due to the gun being damaged during his escape in 2008. Daiki's attempt to cause a time paradox attracts the attention of Reiji Kurosaki, the Time Police's Kamen Rider G Den-O. This sets in motion a series of events that would allow Daiki to complete his plan, by getting his past self accidentally hit by Kurosaki and retrieve the golden gun before it gets damaged, thus putting it into his possession while opening his pursuer's eyes to what he should value more. Though his past self steals the golden gun from him, Daiki obtains a K-Touch from Kurosaki and gains his own Complete Form.
As Tsukasa re-takes command of Dai-Shocker again to hunt down the Super Sentai teams, Daiki confronts Captain Marvelous and Dai-Zangyack hunting down Kamen Rider OOO and have a dispute on which of them would get a treasure. Unintentionally dragging Hina Izumi to the Dai-Shocker headquarters, Daiki meets Tsukasa who asks him to join Dai-Shocker. Daiki declines leaving with Hina and confronting Joe Gibken and Don Dogoier who had just lost their comrades to Decade. Joe recalling what Marvelous told him about Sentai and Riders attacks Daiki while Don and Hina break them up. Calming down after recalling that both sides lost comrades they get on the Gokai Galleon to plan their next course of action. With the idea of meeting Akarenger to prevent more calamities from happening, Daiki reveals a pudding cup he stole from Momotaros to hitch a ride on the Den-Liner to 1974. There they meet Akarenger defeating Baseball Mask and he comes with the heroes. However, once back in the present time, the remaining riders and sentai fight each other with Kamen Rider #1 and Akarenger revealed to only be Decade and GokaiRed in disguise. Betrayed by their former comrades, Joe and Daiki are supposedly killed by GokaiRed and Decade respectively. However, with the revelation that the deaths were just a trick to fool the evil organizations, the riders and sentai fight back. Unfortunately, Daiki actually a true antagonist and revealed everyone was sent to another dimension, so he assembles Big Machine. In Big Machine, Daiki battles Go-Buster Oh until it became augmented by Kamen Rider Fourze's S-Switches and Big Machine gets destroyed. Badly injured, but surviving the explosion Daiki returns Tsukasa's camera and tells him to find his own path while accepting Tsukasa's friendship his way before taking his leave.
Read more about this topic: Kamen Rider Diend