Koichi Sakakibara is the original Kamen Rider Ryuki in the alternate continuity of the Kamen Rider Ryuki Special: 13 Riders. He is a man who previously lived in an apartment where the protagonist Shinji Kido later discovers a Card Deck that leads him into the Mirror World. Sakakibara is revealed to have been the first user of the Ryuki Advent Deck in this timeline, having contracted with the dragon monster Dragreder. His background is that he was drawn into the Rider Fight, but he is characterized by a heroic and determined nature. Unlike some other Riders who participate for personal gain or survival, Sakakibara is motivated by a desire to end the conflict and close the Mirror World permanently. In the story, he plays a crucial role as the predecessor to Shinji. When Shinji is accidentally pulled into the Mirror World by a Mispider, Sakakibara appears as Kamen Rider Ryuki to rescue him. However, Sakakibara is already severely injured from his own battles. After saving Shinji, he attempts to break the Core Mirror to stop the Rider War, but the effort leaves him mortally wounded. With his life fading, he entrusts his Card Deck to Shinji, passing on the mantle of Kamen Rider Ryuki and urging him to continue the fight. This act defines his key relationship with Shinji as a mentor and sacrificial figure. His development is limited to this brief appearance, but his legacy shapes Shinji's journey. As Kamen Rider Ryuki, Sakakibara possesses the abilities granted by the Dragon Advent Deck, including the use of the Drag Saber as his primary weapon. He fights with the balanced power of Ryuki's base form, using his abilities to protect others and challenge the Mirror World's dangers.