Kuuga's World
Ai Yashiro
Ai Yashiro, referred to as Ane-san (sister) by Yusuke, is a detective and the leading operative of the police force formed to fight the Grongi. She mistakes Decade for "Unidentified Lifeform #10" after Decade destroys Baberu. While investigating the lair of the Grongi to take out the resting Gamio, she and her group are overwhelmed by the miasma caused by the Grongi leader's revival. However, Yusuke is not able to save her in time, as the poison she breathed in later killed her. Prior to her death, she makes Yusuke promise that he will fight in order to protect everyone's smiles.
Kamen Rider KickHopper
Sou Yaguruma is a insane young man who transformed into Kamen Rider KickHopper . In Decade, KickHopper is a wild card Kamen Rider who uses a fighting style themed around kicking attacks. He and his partner PunchHopper appear in the World of Kuuga during a dimensional distortion caused by Narutaki to fight Decade. However, when they start focusing their rage on Kuuga, Narutaki causes another dimensional distortion that sends the two to another world. Yaguruma later appears in the World of Decade to prevent Dai-Shocker from taking over, but soon defects to the side of Dai-Shocker.
Kamen Rider PunchHopper
Shun Kageyama is a young man who transformed into Kamen Rider PunchHopper. In Decade, PunchHopper is a wildcard Kamen Rider who uses a fighting style themed around punching attacks.
The Grongi Tribe are a mysterious, ancient civilization that can transform into monsters to kill people for their ruthless game, or Gegeru in the Grongi Language. They are referred to by the police as Unidentified Life Forms and are named in a "Group-Species-Creature type" order.
N-Gamio-Zeda is the wolf-like king of the Grongi Tribe in the World of Kuuga, also called "The Ultimate Darkness". 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 Grongi are attempting to unseal him with a Sacred Gegeru, targeting five police women without spilling blood to revive him. Though they obtain four kills, Tsukasa uses Ai as bait to lure the Grongi out and gives her a nose bleed to ruin the Gegeru. The effects of the distortion revive Gamio anyways, and he uses his power to convert every human corpse within his Miasmic aura into a Grongi. Although his ultimate goal is to have his sired Grongi destroy each other, Gamio is destroyed by Kuuga and Decade performing a combo attack.
In Episodes 1-2, A hermit crab monster of the Me Group, labeled Unidentified Life Form #7. He battles Kuuga and is destroyed by his Dragon Form's Splash Dragon attack.
In Episodes 1-2, A condor monster of the Ra-Group, the judges of the Gegeru, labeled Unidentified Life Form #8. He fights Kuuga and attempts to flee before being destroyed by Kuuga who uses Pegasus Form's Blast Pegasus attack.
In Episode 2, a bison monster that is labeled "Unidentified Life Form #9". Baberu is forced by Decade to reveal the nature of the Gegeru before being destroyed by Decade's Dimension Kick.
In Episode 2, a female sea snake Grongi. She oversees Gamio's revival, she is destroyed by Decade.
In Episode 2, a piranha monster that is one of the leading Grongi 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 Grongi from human corpses using his miasma, but some are later absorbed by him in a last effort to defeat Kuuga and Decade. The remaining Grongi died after Gamio is defeated.
Series:  Kamen Rider Decade