Tsukasa Kadoya is the human identity of Kamen Rider Decade, a young man whose past is largely a mystery, even to himself. He possesses a strong, almost obsessive desire to capture the world through the lens of his camera, yet he is paradoxically a terrible photographer; his pictures never develop properly, appearing distorted and unusable. He interprets this consistent failure as proof that the world itself rejects him, a sign that he does not truly belong anywhere. Despite this personal failing, he claims to be capable of almost any other skill, and his travels through parallel dimensions often prove this true, as he adapts instantly to new roles, such as a police officer, violinist, or chef, with no prior training.

Personality-wise, Tsukasa is often described as hypocritical, self-righteous, and possessing a pseudo-antagonistic demeanor. He carries himself with a confident flair, and is known for a specific gesture of dusting his hands after a fight and delivering a signature catchphrase before a climactic battle: "I'm just a passing-through Kamen Rider, remember that!". This boastful exterior masks a more complex individual, one whose primary motivation is to find his place in the multiverse. He is driven by the need to save his own collapsing world, which is bleeding into nine other parallel universes, each belonging to a previous Kamen Rider. His mission, assigned by the enigmatic Wataru Kurenai, is to travel to these other worlds and confront their Riders. This task creates a central contradiction in his role: he is seen by many as a "destroyer of worlds," a devil who brings ruin, even as his actions often result in saving those very dimensions. He is a being who must destroy the existing Riders to connect and preserve the worlds themselves.

Throughout his journey, Tsukasa forms several key relationships. He is joined by Natsumi Hikari, a girl whose family runs the photography studio where he lives, and who has prophetic nightmares of the Rider War. He also befriends Yusuke Onodera, an alternate universe version of Kamen Rider Kuuga, who chooses to accompany him. His most significant rival is Daiki Kaito, an inter-dimensional thief who transforms into Kamen Rider Diend. Their relationship fluctuates between reluctant alliance and open conflict, as Kaito pursues his own goals across the worlds. Opposing him at nearly every turn is the mysterious Narutaki, a man who is utterly convinced that Decade is the source of all the dimensional turmoil and constantly spreads the rumor of him being a "devil". The arc of Tsukasa's development is a gradual revelation of his own nature. Over the course of the story, he discovers that he was once the Great Leader of Dai-Shocker, a malicious organization bent on conquering all realities. Having lost his memories, his journey becomes not just a mission of salvation, but one of self-discovery, forcing him to confront his own dark legacy and decide what he truly wants to be. He ultimately fulfills the prophesied Rider War, destroying all the other Kamen Riders, only to willingly sacrifice himself to undo the damage and restore the worlds. This act of selflessness redeems him, and he is later revived by the memories of his friends.

As Kamen Rider Decade, Tsukasa wields the power to access the abilities of all previous Heisei-era Kamen Riders. This is achieved through the Decadriver belt and a series of Rider Cards. By inserting a Kamen Ride card, Decade can physically transform into another Rider, such as Kuuga or Faiz, gaining their weapons and fighting styles. Attack Ride cards allow him to execute specific techniques, including his own primary attacks like the Decade Slash (using his Ride Booker in sword mode) and the Decade Blast (in gun mode). His ultimate finishing move in his default form is the Dimension Kick, a flying kick that passes through a series of golden, card-like energy panels to amplify its power. His primary weapon, the Ride Booker, serves triple duty as a card holder in book mode, a gun, and a sword. He also rides a specialized motorcycle, the Machine Decader, which can transform to match the bike of whatever Rider he is currently imitating.

Decade possesses two notable advanced forms. The Complete Form is accessed using a device called the K-Touch, which attaches to the Decadriver. Upon transformation, Decade's helmet displays his own card, and the cards of the nine main Heisei Riders are mounted on his chest armor. In this form, he can summon the final forms of those Riders to fight by his side. The Violent Emotion form represents Decade's transformation into the "destroyer of worlds." Appearing with a more sinister helmet and lighter magenta armor, this form is overwhelmingly powerful and can use the signature attacks of other Riders without needing to physically transform into them first.