Sigma Circular (シグマサーキュラー Shiguma Sākyurā) is a destructive machine created by Banno and Roidmude 004, acting as the mad scientist's will, in order to dominate all of humanity to erase those who opposes his superiority into a data and out of existence from the real world. During the first attempt to activate this machine, Medic was originally meant to be a sacrificial vessel, but Brain takes her place to save her from brainwashing and sacrifices himself instead. Later, Gord Drive and the Grim Reaper Unit infiltrated the Special Situation Defence Center building, and placed Sigma under evolution process before the Kamen Riders (and later joined by the now-defected Roidmude generals Heart and Medic) would stop him from his plan on occurring the second Global Freeze. It kills a weakened Medic while she heals Shinnosuke for his debt to understand the Roidmudes' feelings, and soon being destroyed by Drive Type Tridoron and Heart Over-Evolved Form for good, freeing a data imprisoned civilian and allies alike, even saving Kiriko from being erased into a data.
Sigma Circular is voiced by Ayumi Fujimura (藤村 歩 Fujimura Ayumi) and Masakazu Morita.