Tobei Tachibana
The mentor of the previous Kamen Riders.
Ryoko Mizuki
Keisuke's fiancee, Keitaro's assistant, and a G.O.D. member. She is an Interpol investigator and executes undercover operation in the G.O.D. She became a cyborg by G.O.D.'s operation. Her true identity was exposed protecting a child from Strong Arms Atlas and she was destroyed by the General of G.O.D.
Kiriko Mizuki
Ryoko's twin sister who is an Interpol investigator. She died protecting Keisuke from Strong Arms Atlas's arrow.
A student of Johoku University and Tobei's coffee shop worker.
Chiko's friend and Tobei's coffee shop worker.
Keitaro Jin
Keisuke's father, Keitaro was a robotics genius whose technology attracted G.O.D.'s attention. As a result, Keitaro died saving his son's life at the cost of his own.
Series:  Kamen Rider X