Shinji Kido/Advent Calendar

In the alternate ending, Advent Calendar, Shinji Kido (城戸 真司) was Kamen Rider Ryuki (仮面ライダー龍騎), who was possessed by his deceased twin brother Shinichi Kido and transformed into Kamen Rider Ryuga (仮面ライダーリュウガ) after his Rider suit turned black.

Advent Calendar
In this alternate retelling of the story, Shinji was younger of two identical twins. While Shinji was the kinder and friendlier of the two, Shinichi was aggressive and impulsive. Yet despite of their temperamental differences, the two held a strong bond and were very close.

In a tragic turn of events, Shinichi was killed protecting his brother during an assault by a thug wielding a knife. The killer was in fact someone whom Shinichi had a previous altercation with and his intended target all along. He inadvertently attacked Shinji due to mistaken identity.

Shinji is one of the last surviving Riders in this version of the story. He challenges Asakura to a fight, Asakura using the Survive Shippu card he stole from Ren. Later, Asakura uses the Survive deck's Unite Vent card along with all the Advent cards he's collected from the Riders he defeated, combining all the monsters (along with Dragreder) into Genosurviver.

But because Asakura had not contracted Dragreder, its head starts biting at Venoviper's head, causing Genosurviver to run amok and attempt to destroy Mirror World. Kitaoka kills Asakura and Genosurviver with his Final Vent, but he is also caught up in the explosion and dies as well.

Shinji eventually stumbles across Ren under some rubble, a direct consequence of Ren's previous fight with Asakura. At that moment, Shinji is possessed by Shinichi's spirit, causing his deck, armor and Contract Monster to turn black.

Ren shows up, revealing the Knight under the rubble was a Trick Vent-created copy. Since they are the last remaining Riders, they fight with Shinji as the winner. Odin, who serves to grant the winner's wish in this story, appears and grants Shinji's wish: to revive all the victims of the Rider War.

Forms
In the S.I.C. Hero Saga Kamen Rider Ryuki: Advent Calendar, he was possessed by his deceased twin brother, Shinichi Kido, and his Rider suit turned black along with Dragreder, turning Ryuki into Ryuga and Dragreder into Dragblacker.

Equipment

 * Advent Deck - Transformation device and Advent Card holder
 * V-Buckle - Transformation belt
 * Advent Cards - Trinkets that enable a Rider to summon Contract Monsters, access equipment, and use special abilities
 * Drag Visor - Ryuki's Visor that enables him to use Advent Cards
 * Drag Visor-Zwei - Upgraded version of the Drag Visor used to transform into Ryuki Survive
 * Black Drag Visor - Ryuga's Visor that enables him to use Advent Cards

Vehicles

 * Ride Shooter - Motorcycle used by any Rider to enter the Mirror World