Daiki Kaito, known as Kamen Rider Diend, is a complex figure whose journey is defined by theft, deception, and a hidden moral core. His very name, Diend, is a play on the English words "die" and "end," reflecting his role as an agent who often brings closure or disruption to the worlds he visits. He is introduced as a mysterious and cheerful young man, often to the point of being an annoyance to those around him. This lighthearted facade, however, sharply contrasts with his demeanor in battle, where he becomes serious and focused. His primary stated motivation is a relentless pursuit of material objects he deems "treasures," a quest that drives him to travel across dimensions and often puts him in direct conflict with the protagonist, Tsukasa Kadoya, the titular Kamen Rider Decade. Daiki's surname is a homophone for the Japanese word for "phantom thief," which perfectly encapsulates his role as an inter-dimensional, unpredictable thief.

Beneath the surface of a selfish treasure hunter lies a much deeper and more tragic history. It is eventually revealed that Daiki comes from an Alternate Reality world known as the "World of Diend," where he served as a police officer under a tyrannical entity named Fourteen. In his misguided youth, he believed he was helping to create a better society by hunting down Kamen Riders, whom Fourteen had declared threats to public peace. This changed when he captured the leader of the resistance, Kamen Rider Glaive, only to unmask him and discover it was his older brother, Junichi. Daiki then learned the horrific truth: Fourteen was not reforming people but brainwashing them. Overcome with guilt and a desire to atone for his mistakes, he stole the Diendriver from the evil organization Dai-Shocker, becoming Kamen Rider Diend and a fugitive in his own world. His subsequent journey to other worlds and his obsession with stealing treasures is, in part, a form of self-delusion and a desperate, misguided attempt to gain the power needed to one day return and free his brother.

In the narrative of Kamen Rider Decade, Daiki serves as both a rival and an uneasy ally to Tsukasa. He frequently appears to "help" in battles, often using his summoning abilities as Kamen Rider Diend, only to reveal that his true goal was to steal a specific item from the world or even from Tsukasa himself. Despite his constant self-serving declarations, he develops significant relationships over time. He forms a particular bond with Asumu, a young boy in the World of Hibiki, whom he reluctantly trains and encourages to become a new Oni, showing a paternal and mentoring side he rarely displays. While he initially treats the residents of the Hikari Photo Studio, including Tsukasa, Natsumi, and Yusuke, as mere obstacles or tools, he eventually, albeit lackadaisically, accepts them as friends. He even comes to call Tsukasa a "comrade," though he is never beyond abandoning them without explanation if his own interests are at stake.

His character undergoes a significant development during the story, primarily when he is forced to return to his home world. With the help of Tsukasa and his old friends, he confronts Fourteen and his own brother. The climax of this arc reveals that Junichi was never brainwashed but was a willing collaborator. The two brothers fight, but neither has the will to kill the other, leading to a poignant and unresolved parting. This confrontation forces Daiki to accept his past and his brother's choices, moving him from a state of self-deception to a more honest, if still mischievous, path. His development continues in subsequent appearances, where he continues to operate as a "good" thief, such as in the series Kamen Rider Zi-O, where he returns as an older, arguably wiser, version of himself, still seeking treasures and clashing with new heroes.

As Kamen Rider Diend, Daiki possesses a unique set of abilities that distinguish him from Decade. Using the Diendriver, a gun-like transformation device, he inserts a Kamen Ride card to transform. His primary combat style revolves around summoning other Kamen Riders to fight for him, a power he uses creatively and often to cover his escape. While Decade transforms into other Riders, Diend calls them forth as allies, typically summoning up to three at a time. He also wields a variety of Attack Ride cards that grant him special abilities, such as Invisible for stealth, Illusion to create clones, and Barrier for defense. His signature finishing move, the Diend Blast or Dimension Shoot, involves lining up a series of dimensional card panels to fire a powerful, focused energy shot. Later, he acquires a K-Touch, similar to Tsukasa's, allowing him to access a Complete Form. In this form, his armor displays images of eight movie-exclusive Kamen Riders, whom he can summon simultaneously for a devastating combined attack. True to his nature as a phantom thief, Daiki's powers and methods are all about misdirection, summoning allies to do his work, and always having an escape plan, making him an unpredictable and enduring presence in the Kamen Rider multiverse.