Chikara Saionji (西園寺 主税 Saionji Chikara), previously referred to as a mysterious man (謎の男 nazo no otoko), was a colleague archaeologist of Ryū Tenkūji and Kenjirō Igarashi who aided in their Gamma study a decade ago. Until that day, he betrayed his colleagues after using the Fukami siblings as his guinea pigs to enter the Gamma World, trapping them for 10 years, then sided with Adel, and Alain. Though Chikara expressed regret of his treachery, but deemed Ryū's hope for a better future misplaced as he planned to use the Gamma he summons to his world for his own agenda. Chikara's plans come to fruition after Ghost and Specter manage to collect various Eyecons, as he has five himself, even though he temporarily loses the Billy the Kid Eyecon to Ghost. He then lures the two Kamen Riders into one place, steals their Eyecons, and heads to the monolith below Daitenkū-ji to demand the power to take over the world. However, as he does not have a Ghost Driver worn on his waist, due to being in incomplete state, his plan backfires and he is killed by the monolith's power.