The characters of Kamen Rider Decade exist among various iterations of reality within the series referred to as an A.R. World (Another Rider's/Alternate Reality). The are the A.R. Worlds that are based on the previous entries of the Kamen Rider Series that have aired during the Heisei period of Japanese history. Each differs in some way from the series on which it was based.

World of Kuuga The is the version of reality where Kamen Rider Kuuga takes place. It was the world where Yusuke Onodera originated from before he joined Tsukasa in his journey. The backdrop used to arrive in this world shows a city with police cars and in the background. Within this A.R. World, Tsukasa assumes the guise of a police officer with the talent of understanding and speaking the Gurongi language.Kamen Rider Decade episode "Rider War"Kamen Rider Decade episode "the World of Kuuga"Kamen Rider Decade episode "Transcendence"

Ai Yashiro , referred to as by Yusuke, is a detective and the leading operative of the police force formed to fight the Gurongi. She mistakes Decade for after Decade destroys Baberu. While investigating the lair of the Gurongi to take out the resting Gamio, she and her group are overwhelmed by the miasma caused by the Gurongi leader's revival. However, Yusuke is unable to save her in time because the poison she breathed in later killed her. Prior to her death, she makes Yusuke promise that he will fight to protect everyone's smiles.

Ai Yashiro is portrayed by .

N·Gamio·Zeda is the king of the Gurongi in the World of Kuuga;, he is also called . He refers to himself as otherworldly because he should have never been revived, a fact that even he seems to believe, wondering why he was awakened. He is sealed and the Gurongi are attempting to unseal him with a , targeting five police women without spilling blood to revive him. Though they obtain four kills, Tsukasa uses Ai as bait to lure the Gurongi out and gives her a nose bleed to ruin the Gegeru. The effects of the distortion revive Gamio anyway, and he uses his power to convert every human corpse within his Miasmic aura into a Gurongi. Although his ultimate goal is to have his sired Gurongi destroy each other, Gamio is destroyed by Kuuga and Decade performing a combo attack.

N·Gamio·Zeda is voiced by , who was the narrator for Kamen Rider Kuuga.

Gurongi

The are a mysterious, ancient civilization that can transform into monsters to kill people for their ruthless game, or in the Gurongi Language. They are referred to by the police as and are named in a "Group-Species-Creature type" order. : A hermit crab monster of the Me Group, labeled . He battles Kuuga and is destroyed by his Dragon Form's Splash Dragon attack. : A condor monster of the La Group, the judges of the Gegeru, labeled . He fights Kuuga and attempts to flee before being destroyed by Kuuga who uses Pegasus Form's Blast Pegasus attack. : A bison monster of the Go Group that is labeled . Baberu is forced by Decade to reveal the nature of the Gegeru before being destroyed by Decade's Dimension Kick. : A female sea snake Gurongi. She oversees Gamio's revival, she is destroyed by Decade. : A piranha monster that is one of the leading Gurongi overseeing Gamio's revival. He is destroyed by Kuuga's Mighty Kick when he attempts to intervene in the battle between Kuuga and Decade.

N·Gamio·Zeda is able to create various Gurongi from human corpses using his miasma, but some are later absorbed by him in a last effort to defeat Kuuga and Decade. The remaining Gurongi died after Gamio is defeated.

World of Kiva The is the version of reality where Kamen Rider Kiva takes place. Unlike the original series, this world allows Fangires and humans to coexist, with the King's servants destroying the rebellious Fangires. The backdrop that allows entry into the World of Kiva depicts a nighttime cityscape with Castle Doran emerging from a building. Within this A.R. World, Tsukasa assumes the identity of a violinist with the talent of playing a violin solo similar to those of Otoya and Wataru Kurenai.Kamen Rider Decade episode "Second Movement ♬ Prince Kiva"Kamen Rider Decade episode "The Biting King's Qualifications"

Wataru is a twelve-year-old half-Fangire boy, having the ability to transform into with the aid of Kivat-bat the 3rd. Referred to as the by his servants, Wataru is reluctant to claim the title of of the Fangires. He finds the overseeing of the ceasefire between humans and Fangires a very troublesome and unnerving task due to his fear of making friends and losing control over his Fangire impulses around them. But events leading to the Beetle Fangire attempting to disrupt the peace allows Wataru gets over this fear and takes his place as the official King of Fangires.

He later appears in the World of the Rider War as his reality is absorbed into it. Leading the Riders of his world, Wataru refuses to listen to Tsukasa and Yusuke in uniting all Riders to defeat Dai-Shocker and stop the convergence of the worlds, while in grief over IXA's death. He even went so far to allow the Queen of the Fangires, Yuki, to marry Apollo Geist for the World of Kiva to have an advantage over the other A.R. Worlds, but later changes his mind when the Fangire betrayed them to Dai-Shocker.Kamen Rider Decade episode 30: Rider War: Prologue After defeating Super Apollo Geist, the convergence of the worlds continues and the World of Kiva and Wataru disappear.Kamen Rider Decade episode: "Destroyer of Worlds" However, in Kamen Rider × Kamen Rider W & Decade: Movie War 2010, he is resurrected after Tsukasa has been killed by Kamen Rider Kiva-la. He later appears alongside the Riders of the Nine Worlds to revive Tsukasa and help him battle Super Shocker, assuming Emperor Form with the help of Decade's K-Touch and later uses Castle Doran to fight the Super Crisis Fortress.

Wataru is portrayed by .

Kivat-bat the 3rd

is the third generation of the . He formed a partnership with Wataru to give him the power of Kiva, attaching to the to allow his human partner to transform and use other abilities through the Fuestles.

reprises his role voicing Kivat-bat the 3rd in the character's World of Kiva incarnation.

Arms Monsters

The are servants of Kiva, each the last of his kind. Composed of of the , of the , and of the , they support Kiva by turning into the weapons that allow Kiva to change form. They are absorbed into Beetle Fangire's breast plate and shoulder armor when they question their new king's intentions of removing the human/Fangire coexistence law.

In addition to these alternate versions of the Arms Monsters, the original versions appear in the timeline within the World of Den-O in The Onigashima Warship, where the wayward Urataros, Kintaros, and Ryutaros hide within Jiro (Garulu), Ramon (Basshaa), and Riki (Dogga), respectively until they can be rescued.

, , and , voice Garulu, Basshaa, and Dogga, respectively. Their original incarnations in The Onigashima Battleship are portrayed by , , and respectively.

Ryo Itoya

is a strange young man who is in actuality the . He is the first Fangire that Tsukasa and Natsumi encounter, and reveals to them the truce that exists between Humans and Fangires while posing for a photo in the Hikari Studio.

reprises his role as Ryo Itoya in the character's World of Kiva incarnation.

Beetle Fangire The , whose true name is , is the previous King of the Fangire Race and a member of Tribe. He was exiled for loving a human woman who gave birth to his half-breed successor, Wataru. Seeing the coexistence and peace between the two races as nothing more than a farce for his son's sake, the Beetle Fangire breaks the peace by attacking humans to gather their Life Energy and reclaim his title, even stealing Kivat from Wataru. His actions of removing the law to restart past conflicts between the two races provide the motivation for Wataru to step up and truly claim his place as King of the Fangires. In the end, he is weakened by Decade's Kiva Arrow, then fatally wounded by a Dimension Kick/Darkness Moon Break combo. Revealing that he knew of the Fangire's true identity and intent before he dies, Tsukasa plays a final song for him before the man shatters.

The Beetle Fangire is portrayed by .

The Fangires

The are vampiric creatures that resemble stained glass, originally feeding off of the of humans until they are commanded by their king to coexist with them. However, some of these Fangires are unable to follow this law due to their instincts. Each Fangire belongs to a specific class based on the animal form it takes. Several Fangires also appear in the first episode when Natsumi is trapped within the changing dimensions. They include the Shark, Moose, Horsefly, Rat, Mantis, and Seamoon Fangires. The Mantis Fangire later appears fighting the Beetle Fangire. : A Fangire with the title of , his task is to oversee the progress of all Fangires and their moral conduct, deeming those who are traitors to their king's laws. When the Beetle Fangire regains his title, the Swallowtail Fangire aides his master until he is shattered by Decade's Dimension Slash. The Swallowtail Fangire is voiced by . : A Fangire with the title of , he is chasing after a renegade Fangire when Tsukasa interferes and destroys him with the Ride Booker's Decade Slash. The Lion Fangire is voiced by .

World of Ryuki The is the version of reality where Kamen Rider Ryuki takes place. In this world, there exists the , a battle where the Kamen Riders fight each other within a parallel dimension known as the alongside their for the victor to decide the fate of a defendant in a criminal case. Though no harm occurs to the Rider while in the Mirror World, a battle in the real world can be fatal and use of Vent cards outside a battle is forbidden. The backdrop used to arrive in this world shows Dragredder flying within the Mirror World as shown in a mirrored time display screen on a skyscraper. Within this A.R. World, Tsukasa assumes the identity of a lawyer and the ability to enter the Mirror World without a Card Deck.Kamen Rider Decade episode "Battle Trial: Ryuki the World"Kamen Rider Decade episode "Super Trick of the Real Criminal"

Shinji Tatsumi is a young cameraman who works for the Atashi Journal, a popular magazine that covers the Kamen Riders and the trials. When the editor of the journal is murdered, Shinji becomes a Kamen Rider to decide the fate of the suspect. Like his namesake in Ryuki, Shinji has the ability to transform into and use the power of his Contract Monster Dragredder. He later appears in Kamen Rider × Kamen Rider W & Decade: Movie War 2010 alongside the other Riders of the Nine Worlds to help in reviving Tsukasa and helps him battle Super Shocker, assuming Ryuki Survive with the help of Decade's K-Touch and later into his Ryuki Dragredder form with the All Rider Final Form Ride Card to fight the Super Crisis Fortress.

Shinji Tatsumi is portrayed by .

Ren Haguro is a former journalist of the Atashi Journal and Shinji's former best friend, mysteriously leaving the group three years ago without explanation. Like his namesake in Ryuki, Ren has the ability to transform into and use the power of his Contract Monster Darkwing. When Reiko is murdered, Ren enters the Trial to find the Rider who possesses the Time Vent Card. After defeating Scissors, Ren encounters Decade in a brief confrontation before Zolda intervenes. Later while forced to fight Shinji who accuses him of playing a part in Reiko's death, Decade's fight with Imperer stops theirs. Soon after, Decade battles Knight with the former being the superior fighter until Abyss intervenes. After founding and defeating Odin, Ren acquires the Time Vent card at the cost of being too injured to use it himself, shortly when Abyss is revealed to be Reiko's murderer, with Shinji takes Ren's place, accompanied by Tsukasa to prevent Reiko's death from the hands of Kamata.

Ren Haguro is portrayed by .

Reiko Momoi