Natsumi Hikari
Tsukasa Kadoya's traveling partner, she lives and works in the Hikari Studio with her grandfather, she has lived there since she was little. Before the adventures began, Natsumi was a member of the Taigaku Club or TG Club, a group of dropouts who got bored with school and wanted to find out their own answers. But they never carried out their intent and returned to school, promising to meet again years later. Natsumi begins to have nightmares of Decade prior to finding Decade's belt and giving it to Tsukasa. Fully aware of world's endangerment, Natsumi decides to accompany Tsukasa on his journey to prevent her dream from coming true. She occasionally keeps Tsukasa in line with the Hikari Family Secret Laughing Pressure Point, which makes someone instantly laugh. She also appears to have cares for Tsukasa's well-being. Despite Narutaki's offer to save her, she refuses and chooses to see Decade's journey to the end. However she starts worrying during the time in the Hibiki World after hearing from Narutaki that Decade's actions will actually destroy all worlds. While in the Shinkenger World, Natsumi assures Tsukasa that the Photo Studio is his home before her near-death experience by Apollo Geist in the Black RX World, learning that Tsukasa used some of his life to save her later. It is only after their adventures in the Amazon World that Natsumi ponders if there's a point behind their journey.
Eijiro Hikari
He runs the Hikari Photo Studio and Natsumi's grandfather. When he discovers that his photo studio is the means of accessing the A.R. Worlds by accident, he is not surprised but amused at the fact and is glad to assist Tsukasa in his mission. Eijiro is also the most easygoing of the group and gets along well with Kivara. In the Decade World, Eijiro is revealed to be a guise assumed by Shocker's Doctor Shinigami, an executive member of Dai-Shocker from Switzerland. Doctor Shinigami has the ability to assume the form of the Inhumanoid Ikadevil, a squid monster, which he claims is his greatest creation. Though seemingly killed in the final battle by Decade, Diend, and Momotaros using several other Riders as weapons, Shinigami resumes his Eijiro guise, who seemingly forgets ever being Doctor Shinigami to begin with. In the Kamen Rider × Kamen Rider Double & Decade: Movie War 2010, he transforms into Super Doctor Shinigami, the leader of Super Shocker against his will using the black Shinigami Hakase Memory, given to him by Ryubee Sonozaki. He revives the Neo Organism to defeat Kamen Rider Decade once and for all. However, he reverts back to Eijiro as the Memory ejects out of him caused by Kamen Rider Double's attack. He is saved by Natsumi in her Kamen Rider Kivara form. Afterwards he returns to the studio with the others once Ultimate D is destroyed by Double and Decade.
Wataru Kurenai / Kamen Rider Kiva
Wataru Kurenai is the original Kamen Rider Kiva who represents the others by giving Tsukasa his mission while they freeze Natsumi's world in place to give him time to stop its destruction. He later appears in the World of the Rider War to fight Decade after telling him his mission was to destroy the Riders in the Nine Worlds rather than befriend them.
He is a mysterious man in a brown trenchcoat who refers to himself as a prophet, and is protected by an aura that renders him immune to the convergence of the ten worlds. He has the ability to move freely between the worlds and summon Riders from other worlds. He firmly believes that Decade shouldn't exist in any world and tells the other Kamen Riders that Decade will destroy their world. He also desires to save Natsumi, offering her freedom while telling her that the Rider War will awaken Decade's true nature. After Decade defied his premonitions of doom in the previous nine worlds, Narutaki decides to personally deal with him in the Hibiki World by summoning a Bakegani, revealing to Natsumi that Tsukasa's actions are actually destroying the worlds rather than saving them as they were told, and that only she can stop the madness now. However at the Negatives World, Narutaki expresses his joy over Decade finding himself a world which he can exist in. In the World of the Rider War, he asks for Tsukasa's help to stop Apollo Geist from speeding up the destruction of all worlds, but this doesn't change his belief that Tsukasa is the true cause of the destruction. In Kamen Rider Decade: The Last Story, Narutaki becomes Colonel Zol, an executive member of the evil Super Shocker organization. However, when Doras fell from the Super Crisis Fortress and knocks him out of his way, he reverts back to Narutaki and curses Decade for causing a new calamity born from the Neo Organism's antics before running off. His current whereabouts are currently unknown.
Kivara is a white female Kivat originally from the the Kiva World, where she is Kivat-bat the 3rd's younger sister. She is tiny enough to be hidden in an adult's closed fist. She learns of Decade from Narutaki, and uses her power to bring Yusuke into her world, and then sends Tsukasa into another to do battle with Kaixa. She later ends up traveling with the Hikari Studio, though remaining in contact with Narutaki with only Natsumi aware of it. She seems to like Yusuke the most out of the traveling group as she is delighted when he returns to travel with them after they leave the Agito World. In Hibiki World, she assists Ohja in summoning a Bakegani to be used on Decade as Gyuki was not getting the job done, placing her overall goals further into question. In Kamen Rider × Kamen Rider Double & Decade: Movie War 2010, Kivara helps Natsumi to become Kamen Rider Kivara.

Apollo Geist
The first member of Dai-Shocker Tsukasa confronts, Apollo Geist is the Security Chief of Dai-Shocker, hastening the worlds' convergence to make invasion easy for his organization. He normally assumes the form of a man called Guy until he invokes "Apollo Change". Originally from the World of X-Rider, he was revived by the Government Of Darkness after being killed by X-Rider. However, his lifespan is short as a result and thus uses the Perfecter to take the life force out of people to augment his own. When the Perfecter is stolen from him by Diend and then destroyed by Decade, Apollo Geist eventually comes up with the plan to marry Yuki, the Fangire Queen, becoming a Fangire as a result. As a result of his transformation, he now becomes Super Apollo Geist with the ability to remove other realities from existence. Using his power, he revives the monsters from the Nine Worlds. He attempts to make Natsumi his bride after Yuki is destroyed by Decade Complete Form. Super Apollo Geist fights Decade and Diend until Kiva and Hibiki distract Super Apollo Geist long enough for him to be mortally wounded by Decade Complete Form with Diend's power, Geist swearing that he will 'revive as the greatest nuisance in the universe' before he is destroyed.
Series:  Kamen Rider Decade