Daiki Kaito is a cocky young man who can be 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 seems to know about Tsukasa's past, referring to sea cucumbers as being something Tsukasa had always wanted to taste. He travels from dimension to dimension, stealing what he refers to as 'treasures'. It is revealed that he comes from an A.R. World referred to as Diend World, 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 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 had stole the Diendriver from Dai-Shocker and became Kamen Rider Diend, 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 and finally frees it from Fourteen's tyrannical 'peace', and settles matters with his brother. He entered Shinkenger's World to steal Shinken Gold's Ika Origami, but in fighting an Ayakashi, his Diendriver was stolen and he was forced to trade it for the Ika Origami after the Shinkenger recovered it. 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.

By using the Diend Kamen Ride Card in the Diendriver and then shooting the Diendriver above him, Daiki becomes Diend. Diend can move extremely fast when he personally fights rather than use his Kamen Ride cards through the use of the microwave-emitting Tesla Bands around his wrists. In the Kamen Rider × Kamen Rider × Kamen Rider The Movie: Cho-Den-O Trilogy's Episode Yellow: Treasure de End Pirates, Diend gains access to his own Complete Form, similar to Decade's Complete Form. He wears the Diend Complete Kamen Ride Card on his helmet, embedded in the Diend Crown, and on his chest are the Kamen Ride Cards of the main antagonistic Kamen Riders from the Heisei Kamen Rider films (excluding those from the Den-O films).