List of Kamen Rider Decade characters in the Nine Worlds

The characters of Kamen Rider Decade exist amongst various iterations of reality within the series referred to as an A.R. World (Another Rider's/Alternate Reality World). The Nine Worlds (9つの世界, Kokonotsu no Sekai) 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.

The World of Kuuga (クウガの世界, Kūga no Sekai) 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 Mount Hitoki (灯溶山, Hitoki-yama) 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.

Ai Yashiro (八代 藍, Yashiro Ai), referred to as "Big Sis" (あねさん, Ane-san) by Yusuke, is a detective and the leading operative of the police force formed to fight the Gurongi. She mistakes Decade for "Unidentified Life Form #10" (未確認生命体十号, Mikakunin Seimeitai Jūgō) 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 in order to protect everyone's smiles.

Ai Yashiro is portrayed by Hiroko Sato (佐藤 寛子, Satō Hiroko).

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