Doctor D is a scientist who works for Gel-Shocker and an antagonist in the Kamen Rider franchise.
Doctor D analyzed Takeshi Hongo's and prepared a trap to immobilize him. Gel-Shocker initiates his plan, but they are foiled when Hayato Ichimonji returns and saves Hongo.
Two years after Gel-Shocker's demise, Doctor D was developed a way to manipulate history. However, an explosion erupted in his base and killed him.
However, his cells were saved by the Great Leader of Shocker and given to Roidmude 089 when he allied with Shocker. Using his cells, 089 assumed the form of Doctor D and used his research to create the History Modifying Machine.