Kamen Rider Diend is the secondary Rider in the series Kamen Rider Decade. His name comes from the English words "die" and "end", as well as "the end".
Daiki Kaito 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.
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 becomes 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.
World of Blade
Though he briefly appeared in Blade's World, revealing he knows Tsukasa, Daiki found nothing worth taking.
World of Faiz
Arriving in Faiz's World ahead of Tsukasa, Daiki witnesses Faiz's battle with the Butterfly Orphnoch and was unable to catch him to acquire his Faiz Gear. When Tsukasa arrives in Faiz's World, Daiki reveals that Faiz is among the student body and warns him not to meddle in his affairs. However, as Tsukasa decides to find Faiz, Daiki gets Lucky Clover's aid by showing them the photograph that Faiz dropped during the fight under the assumption he wanted to join their group, leaving them to attack Yuri before Takumi becomes Faiz to protect her. Daiki then attempts to run away from school, but realizes that he needs to stay and make sure the belt is in proper hands.
When the Orphnochs acquire the belt, Daiki reveals his true intentions as he becomes Diend. He kills the Centipede Orphnoch before he and Decade fight over the Faiz Gear until Takumi regains it, later disposing of it. But in spite of being told Takumi had something more precious than the Faiz Gear, Daiki takes the Ride Booker from Tsukasa in an attempt to take the Faiz Gear from him. Narutaki whisks Tsukasa away to fight Kamen Rider Ryuga, but Daiki interrogates Kiva-la on the location to save Tsukasa, claiming that the Faiz Gear is what he came for as he gives Tsukasa back his Ride Booker.
However, during the Lucky Clover invasion of the Smart Brain School, Daiki returns once again to Tsukasa and Takumi's aid, returning the Faiz Driver to Takumi. After the fight, Daiki claims the Orga Gear for himself out of rubble while seeing it as the "more precious treasure" Tsukasa spoke of, leaving a little note on the painting of Agito's World as a warning not to meddle in his affairs again, which further irritates Tsukasa.
World of Agito
In Agito's World, Daiki is training along with Yusuke and others to become the next G3-X user. Acquiring the position, it is only revealed later that Daiki's real goal in Agito's World is to steal the newly created G4 chip that was stored in Ashikawa's locker, Yusuke confronts him until Toko arrives and blasts the chip to bits. Daiki later aided Yusuke in protecting Ashikawa, only to find out what he possessed to be more precious than the G4 Chip. However, seeing that to be his Agito powers, Diend leaves the fight scene to the next world as he could not take it with him.
World of Den-O
In Den-O's World, Kaito attempts to coax the Imagin Momotaros, who took Yusuke's body, into allowing him to use a Final Form Ride card on him to become a DenLiner. When M-Yusuke refuses, he is fought ragged by Diend until he leaves him to reconsider his offer.
However, when he offered again, Decade takes the card from Diend while defending Momotaros before helping him regain his self and form. Sticking around Den-O's World, Diend sneaks aboard the DenLiner to the past where his summoned Riders were used by Urataros, Kintaros, and Ryutaros.
World of Kabuto
In Kabuto's World, interested in the Clock Up ability of the Kamen Riders, Diend saves Decade from TheBee and Gatack, fighting the latter by himself as he is further impressed by the Clock Up System and proceeds to get it from Kabuto. But after the attempt blew up in his face, Daiki receives a "treasure" from Tsukasa which was actually a vial of pepper.
World of Hibiki
In Hibiki's World, after an attempt to rob Hibiki, Daiki learns from Asumu of the three scrolls owned by the three Oni schools. Working his way in Zanki's school, he makes a preposition to aid him in settling things with Ibuki and his school. However, during the fight with Decade, Diend reveals his true colors after stealing the Zanki and Ibuki school scrolls and having Momotaros cover his escape. He then admits to get the last scroll from Hibiki who blocks his path and gives him his Onkaku to give to Asumu along with his final request. The next day, Daiki uses Asumu to distract Hibiki lost enough to steal the scroll, invoking the man's change into Gyuki to cover his escape. However, after reading the scrolls, Daiki returns to give Asumu the Onkaku in order to see the harmony of the three Ongekidō styles. He takes his leave soon after, though thanked by Asumu and the others in spite of his protest that his motives were selfish.
World of Negatives
He is seen again in the Negative World, following Tsukasa before he overheard Otoya speaking of treasure that attracts the thief into fighting alongside Decade against the Dark Riders. Though he gained the treasure (through the use of paper airplanes), the K-Touch, Diend loses it during the fight.
World of Diend
Returning to his home world, Daiki meets up with his old friends Haruka and Shin as they fight Darkroaches as Larc and Lance, the latter blaming him for Junichi's brainwashing. Tsukasa approaches Daiki after Natsumi and Yusuke have also been brainwashed by the evil Fourteen, and Daiki agrees to help Tsukasa, Haruka, and Shin defeat Fourteen and his brother who can transform into Glaive. Together, they reveal Fourteen's true form as Jashin 14 and destroy him, freeing everyone from brainwashing. However, Junichi revealed he was never under Fourteen's control and was working for him on his own free will. Diend and Glaive fight, though neither has the will to kill the other, especially after Tsukasa tells them that brothers must look after each other, no matter what.
World of Shinkenger
Junichi and Daiki go their separate ways as Daiki travels to the next world, with the intent to obtain the Shinkengers' Origami. Starting with Genta Umemori's Ika Origami, Daiki runs off with Genta in pursuit. Deciding to fight him as he cannot lose him, Daiki gets an unexpected visit from Chinomanako who steals his Diendriver and hounds him until he makes his way to Tsukasa who has Yusuke take him back to the Hikari Studio. He eventually gets it back from ShinkenGold when Chinomanako is destroyed by the combination of Decade, Kuuga, and the Shinkengers in exchange for the origami he stole.
World of Black RX
In BLACK RX's World, Diend's next attempt treasure is the Perfecter from Apollo Geist, whom Tsukasa used it to restore Natsumi into life.
World of Decade
Daiki joins the Rider Battle Tournament as one of the final winners, he shows himself and telling Tsukasa is too rush and makes Daiki only able to steal GiGi Armlet from the defeated Kamen Rider Amazon. When the tag team battle start, he tags with Tsukasa and Yusuke to fight Kamen Rider V3, Super-1, and Black only to run away by using Attack Ride Invisible card after delivering Attack Ride Blast card to attack to all participants. He later infiltrate Dai-Shocker with disguising as one of the Shocker trooper, show himself when protecting Natsumi and Narutaki from Shockers attack. When Narutaki makes a suggestion to call all the surviving riders to stop Dai-Shocker, Daiki makes a stupid decision to invite Kamen Rider Ouja and KickHopper to join alliance with him. Of course due of their wild nature, they are willing to join Dai-Shocker instead join him to destroy them, when some of the Dai-Shocker armies appeared, Daiki and Natsumi are halted by Ouja and KickHopper when they want to make an escape. Daiki transforms into Kamen Rider Diend and summons Kamen Rider Gai, Raia, and PunchHopper to hold Ouja and KickHopper, make them finally managed to escape. However, General Jark overwhelmed him and before Daiki is finished, Tsukasa comes in aid. After General Jark is pinched, Daiki uses Dimensional Shoot to destroy the armies. Daiki and Tsukasa later heads to Dai-Shocker castle, only to be thrown by Ikadevil and Garagaranda with the rest of the surviving Dai-Shocker members. Nearly lost their winning chance, all previous Riders are coming in aid. In battle, he later finished off Ikadevil with Diend Edge tagged along with Tsukasa using Decade Photon and Momo Fang finisher, he is later being blown away by Onodera in Kamen Rider Kuuga Rising Ultimate Form Pyrokinesis attack and shows up again on Kamen Rider J's Jumbo Form shoulder, claiming that it is his other hidden treasure to defeat King Dark. When J is defeated easily, Daiki uses Final Form Ride: Decade into Tsukasa so he transforms into a giant size of Decadriver and attached into J's waist, transforming him into Kamen Rider Decade Jumbo Formation Complete Form. After the battle, he returning Kamen Rider Amazon's GiGi Armlet and being claimed as friend by Amazon. At the end, he tells Tsukasa that it's not just his journey is his world, but also to all of them.
World of Amazon
In Amazon's World Diend appears again looking for the GaGa Armlet. Diend manages to infiltrate the Dai-Shocker base and learns the armlet is in the possession of Llumu Qhimil. He joins Decade and Amazon in a fight against the ten faced demon and after GiGi Armlet is stolen from Amazon, Narutaki presents Kaito with a card that will make Diend stronger on the condition that he defeats Decade. Kaito takes the card but says that there are other options as well. Kaito then appears to tell the gang that the boy who stole the GiGi Armlet, Masahiko is being subjected to Dai-Shocker's Human to Monster project. Diend then battles with Decade and Amazon having recovered the GiGi Armlet and manages to steal the GaGa Armlet from Llumu Qhimil until Decade uses an Attack Ride Card to take it from him. The riders then destroy Llumu Qhimil and Diend joins the Hikairi Photo Studio as a traveling partner into another world. They end up in the worst possible world as they enter the World of the Rider War from Natsumi's nightmare.
World of the Rider War
In the World of the Rider War, Diend helps Asumu kill off Saga and his Fangire minions. Asumu explains to Daiki that there are some treasures that should never be taken. When Tsukasa goes to save Natsumi, Kaito appears to tell him about the trap Super Apollo Geist has planned but Tsukasa goes anyway, promising Kaito that if he dies Kaito can have the world. Diend helps Tsukasa fight Super Apollo Geist and the revived Kaijin even giving him a Diend Final Attack Ride to destroy Geist. But when the Rider War starts, he shoots Decade at point blank range marking future battles.
Movie Wars 2010
In Kamen Rider × Kamen Rider W & Decade: Movie War 2010, Decade Final Chapter, he tries to convince Natsumi to destroy the photographs of Tsukasa as they are only memories of who he used to be as Tsukasa has now accepted his fate as the Destroyer of Worlds as Decade Violent Emotion.
After Decade is destroyed, he and Natsumi meet Wataru Kurenai who explains Decade's role as a substitute for the riders of the A.R. Worlds. When Tsukasa is revived, Diend joins the fight against Super Shocker. After the struggle he is seen with thegang posing for a picture.
Kamen Rider × Kamen Rider × Kamen Rider The Movie: Cho Den-O Trilogy
Diend reappears in Kamen Rider × Kamen Rider × Kamen Rider The Movie: Cho Den-O Trilogy. In the film Daiki allows himself to become possessed by a Relict Imagin in order 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 on that date in the past.
He allows the Spider Imagin to be destroyed and hitches a ride back in time on the DenLiner to attempt to cause a time paradox so he can be arrested by 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. Although his past self steals the golden gun from him and escapes, Daiki obtains a K-Touch from Kurosaki and gains his own Complete Form and destroys Kamen Rider G Den-O.
OOO, Den-O, All Riders: Let's Go Kamen Riders
Kamen Rider DiEnd was part of a group of reinforcement riders led by Akira Date/Kamen Rider Birth to aid the primary riders in defeating the Rock Great Leader. All the Kamen Riders got on their bikes and performed the All Rider Break attack which destroyed the Great Leader.
Kamen Rider × Super Sentai: Super Hero Taisen
Daiki and Tsukasa both appeared in a movie alongside the Gokaigers in a Super Sentai meets Kamen Rider, Kamen Rider × Super Sentai: Super Hero Taisen movie. 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 was not pleased about being tricked, so he assembles Big Machine in revenge against Tsukasa. 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.
Kamen Rider Taisen
He's one of the few Heisei Riders who is massacred by the Showa Riders.
Kamen Rider: Battride War
Kamen Rider Diend appears as a Second Rider in the video game Kamen Rider: Battride War.