Kimon Tanaka was a sea captain who hunted dolphins for a living. He captured several dolphins for the Shinobi research centre, one of whom was Porm (Aquaman’s Dolphin Mother). When Kimon’s boat was destroyed he was left by Aquaman to be eaten by sharks. However, Kimon was rescued by his brother Tanaka, head of Raiden Industries. Tanaka rebuilt his brother’s body, turning him into a cyborg.
Dubbing himself Demon Gate Kimon set out for vengeance against Aquaman.