|This article is about a character in Kamen Rider Decade.|
Sayo Kadoya (門矢 小夜, Kadoya Sayo) is Tsukasa's younger sister. She is a pianist and is protected by Nobuhiko Tsukikage due to her possession of the Stone of the Earth, which is the source of Dai-Shocker's travel between the A.R. Worlds. This allows her to become High Priestess Bishum (大神官ビシュム, Daishinkan Bishumu), whom Shadow Moon manipulated to hate Tsukasa and turn Yusuke into her puppet. After realizing that she has been used, she destroys the Stone of the Earth and later makes amends with Tsukasa before he leaves.
In the manga adaptation of All Riders vs. Dai-Shocker, based on the original script when the movie was planned to be the original ending of the TV series, when Tsukasa gets through to Sayo, Shadow Moon prepares to kill her but BLACK RX intervenes and it's he that smashes the Stone of the Earth. Shadow Moon knows who RX is. In the movie, Tsukikage was clearly enraged when Sayo destroyed the Stone of the Earth, but immediately focused his attention on Decade and Kuuga, leaving the Dai-Shocker Castle to fight them while Sayo was next seen at the end of the movie making amends with Tsukasa in the Hikari Studio as she begins her own journey.
At the end Tsukasa wonders if he has found his place as a Rider and Sayo tells him to "go find the answer". The main characters apparently leave with the other Riders.
- Aurora Curtain (オーロラカーテン): Sayo's powers that manifested as a silver wall with the ability to traverse time and space. Originally she was unable to use it for her own travel. It is unknown if she was able to use it later on her own adventures.
- Mind-Control (Formerly): By using the powers of Stone of Earth, Sayo turning Yusuke Onodera into Kuuga Rising Ultimate that turn him into her puppet against Tsukasa. She lost this ability and freed Yusuke from her control when she destroyed the Stone of Earth.
- Stone of Earth (地の石) (formerly) - a stone wielded by Sayo as Bishum.
Behind the scenes
Sayo Kadoya is portrayed by Moe Arai (荒井 萌, Arai Moe).
- Sayo was supposed to join the main cast after the movie, but for unknown reasons, she didn't.
- Aurora Curtain in Kamen Rider Official Website