1. "This television special is an alternate telling of the Kamen Rider Ryuki story." 13 Riders is an alternative timeline, not following the Kamen Rider Ryuki series.
2. The only known contestants are Kido, Akiyama, Sudo, Kitaoka, Tezuka, Shibaura, Asakura, KROdin, KRTiger, KRImperer, Takamizawa, Kirishima, and former Ryuki (Sakakibara). So we can assume that Sakakibara is the first contestant expelled. Do you get it?