Dark Shinji

Dark Shinji is the Mirror World counterpart of Shinji Kido and the main antagonist of the movie Kamen Rider Ryuki: Episode Final.

Kamen Rider Ryuki: Episode Final
When she was very young Yui Kanzaki met and befriended a young Shinji Kido. Despite Shinji promising to play with her again, he never did and so a lonely Yui subconsciously created a duplicate of Shinji in the Mirror World.

Several years later, the Dark Side Shinji clone would become Kamen Rider Ryuga and join the Rider War in order to enter into the real world and become a real person by killing his original counterpart.

As Ryuga, Dark Shinji saved Miho from Kamen Rider Ouja and convinces her that he was the real Shinji. Dark Shinji would later take the real Shinji's place on a date, but Miho eventually realized he was a fake. Dark Shinji then reveals he is Kamen Rider Ryuga and attacks Miho, who transforms into Kamen Rider Femme to fight back. Ryuga fatally wounds Miho, but Shinji shows up at that point to save her.

Shinji finally realizes who his dark counterpart is and blames himself for the creation of Dark Shinji. Ryuga takes advantage of Shinji's guilt to trick him into unifying their bodies so he can take over Shinji's body and become a real human. Ren then transforms into Kamen Rider Knight and battles Ryuga, but Ryuga defeats Knight. Ryuga then prepares to finish off Ren but Shinji resists Ryuga internally, resulting in them separating. Shinji then fights his dark doppelganger in a climatic battle, and finishes him off with Ryuki's Final Vent.

Kamen Rider Zi-O
Learning of Shinji's dark counterpart that was defeated in the Rider War, Uhr travels to the Mirror World to meet him and gives him the Ryuga Another Ridewatch, enabling him to transform into an Another Rider.

The Another Rider fights Sougo and Geiz and manages to overpower then by reflecting back their attacks. Geiz attempted to ask White Woz to help them defeat Another Ryuga, but White Woz's solution was to attack the real Shinji instead, reasoning that if Shinji was gone then Dark Shinji would disappear. Dark Shinji then transforms into Another Ryuga and fights White Woz, succeeding in overpowering him.

After Another Ryuga became bored with fighting, he leaves to return to Mirror World. but Sougo attempts to pull him back but this only leads to Sougo being dragged into the Mirror World along with Ryuga.