Tsukasa Kadoya (門矢 士, Kadoya Tsukasa?, portrayed by Masahiro Inoue) is a hypocritical and self-righteous young man who does not know much about his past. He has a strong desire to "catch the world" using his photos, but they never develop well. He sees this as a sign that he is not of this world, unaware of it being a prelude of it being supplanted by the worlds of the other nine Kamen Riders. However as Decade, he is seen as an agent of destruction, usually being called "the devil" (悪魔, akuma?), a rumor spread by Narutaki, who believes Tsukasa does not belong in any world even his own. In the World of Kuuga, he is briefly called "Unidentified Lifeform 10". It is also revealed that those who appear in his badly developed photographs have some fate befall them, be it death or otherwise. As Tsukasa travels between dimensions, his clothing changes to suit a specific role in the Rider's World, including some accouterments similar in color to his camera and Decade's armor. He also exhibits some prior knowledge or a talent necessary to his quest in the Rider's Worlds he enters. Some examples are his knowledge of the Gurongi language in the World of Kuuga and his enrollment as a student in the World of Faiz. He has also mentioned before that he is capable of doing anything except taking photos. According to Wataru Kurenai, Tsukasa's purpose is to be whatever the other A.R. Worlds needed him to be in order to save the A.R worlds: the hero, the villain, the destroyer, or the savior.
In the film Kamen Rider Decade: All Riders vs. Dai-Shocker, it is revealed that Tsukasa was the Great Leader of Dai-Shocker (大ショッカー首領, Daishokkā Shuryō?), an evil organization that is attempting to take over all dimensions. He commissioned the creation of the Decadriver in order to prevent the collapse of the A.R. Worlds and allow Dai-Shocker to conquer them. Due to circumstances not covered in the film or the television series, Tsukasa suffers from amnesia and ends up in the A.R. World that is the home of Natsumi Hikari, who returns the Decadriver to him when the distortions manifest in her world and allow Tsukasa to resume his mission, without any knowledge of the intended use of the belt as he slowly regains his memories. Finally, after fighting Dai-Shocker through the alternate worlds, he and Diend kill the last of the Dai-Shocker's active Leaders, Super Apollo Geist.
However, it is eventually revealed that his true mission is to destroy the other A.R. World Riders and their worlds, a role he undertakes as the Rider War commences, becoming Kamen Rider Decade Violent Emotion. While he destroys all the other Kamen Riders except for Diend, Natsumi uses Kiva-la, a tiny Kivat that the group encountered in the World of Kiva, to transform into Kamen Rider Kiva-la. Decade then willingly allows Kiva-la to impale him through the Decadriver, causing Tsukasa to die, but not before giving Natsumi the cards left behind when he destroyed the other Kamen Riders. His death is the catalyst for the restoration of all the A.R. Worlds and everyone who died in the Rider War, even those Decade did not kill personally, such as Kamen Riders Leangle and Ixa. Afterwards, the memories of his friends are used to revive him and he leads the other Kamen Riders in the final battle with the newly formed Super Shocker and the revived Neo Organism. After a long, fierce battle, he teams up with Kamen Rider Double to destroy the Super Crisis Fortress and use the Triple Extreme Kick to destroy the Neo Organism in the form of Ultimate D. Afterwards, he and his friends return to the Hikari Studio to continue in their journey across the A.R. Worlds.
In Kamen Rider × Super Sentai: Super Hero Taisen, Doktor G resurrected and gathered all defeated Rider villains, recreating Dai-Shocker. He told Tsukasa that the Riders and monsters needed to destroy the Super Sentai to ensure the survival of the Riders, and, answering that, Tsukasa took control of the new Dai-Shocker, becoming its Great Leader again to battle the Super Sentai and also the Dai-Zangyack, a new union of Super Sentai villains led by the Gokaiger's Captain Marvelous. As the Great Leader of Dai-Shocker, Tsukasa apparently destroyed many Super Sentai teams, apparently leaving only GokaiRed and GokaiGreen alive, while Decade himself was seemingly the last remaining Rider. After he was wounded in his battle against Gokaired, Doktor G used the opportunity to overthrow him and take control of the organization. The story about Riders needing to destroy Sentai to survive was a lie, and Doktor G was actually allied with Dai-Zangyack and using both GokaiRed and Decade to destroy the other heroes and afterwards kill the nominal leaders of the two organizations. However, Tsukasa and Marvelous had realized that trap. The destruction of the Super Sentai actually just an act and the truth was, all the Super Sentai and Riders were just sent into a rift between dimensions. Tsukasa turned against Dai-Shocker and Dai-Zangyack, supported by all other Riders and Super Sentai. Alongside the Gokaiger, he killed General Shadow. In the end, he faced Doktor G, the man behind the new Dai-Shocker, and defeated him with the help of the Super Sentai team Goseiger, and Kamen Riders Ryuki and Blade. However, after being defeated, Doktor G was revealed to be Decade's old enemy, Narutaki, who escaped afterwards, and his real identity was left a mystery.
Often, before a climactic battle begins, a major villain will ask a question to know who Tsukasa is (always phrased as "You bastard...who do you think you are?!" (貴様、何者だ？, Kisama, nani mono da??)), to which Tsukasa answers "I'm just a passing-through Kamen Rider, remember that!" (通りすがりの仮面ライダーだ、覚えておけ!, Tōrisugari no Kamen Raidā da, oboeteoke!?). This phrase is also a reference to Kamen Rider Kuuga's first Episode, In which Yusuke Godai (Kuuga) meets Kaoru Ichijo, who asks what is his occupation, answering by saying that he's just a Passing-Through Dream Chaser.
Tsukasa uses a "Decade Pink" Blackbird, fly 135 Twin Lens Reflex Camera.
Read more about this topic: Kamen Rider Decade (character)