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Simon Marcus is a UN official who is secretly the leader of the Downfall terrorist organization and the secondary antagonist of the Kamen Rider Build V-Cinema movie Kamen Rider Build NEW WORLD: Kamen Rider Grease.
He was portrayed by Michael Tomioka.
As a representative of the UN, Simon Marcus met with Japan's Prime Minister Taizan Himuro to discuss the Rider System. Also present was Taizan's son Gentoku Himuro who acted as a translator. While Taizan and Gentoku stated that the Rider System was essential for Japan's defense, Simon Marcus expressed concern for what would happen if Downfall obtained it.
Gentoku then led Simon Marcus into another room where Shinobu Katsuragi showed the Build Driver to Simon. Just then, the meeting was attacked by a group of Downfall operatives led by Keiji Uraga, who had hacked into the Guardians protecting the meeting. Gentoku transformed into Kamen Rider Rogue to fight them off, but Simon Marcus assumed the form of Phantom Crusher and disabled Gentoku's transformation, revealing himself as the leader of Downfall. With Gentoku unable to transform, Simon had him tied up and brought to another room.
Later on, Simon was killed by Keiji Uraga, who sought to usurp his position.