Giant Devil is a gigantic robot originally designed by Professor Gill to be his ultimate weapon and a major antagonist in Kikaider 01.
The blueprints for Giant Devil were first drafted by Professor Gill, who tattooed them on the backs of both his children to prevent anyone else from getting their hands on them. Gill would later perish before he could get the chance to create Giant Devil, but others with knowledge of the blueprints and Giant Devil would seek out the boys in order to use the blueprints on their backs to build the robot so they could use it for world domination.
Both the Hakaider Squad and SHADOW organization sought out Gill's son Akira in order to complete the Giant Devil, but the boy was protected by Kikaider 01. Later on, SHADOW was able to capture Hiroshi and get the blueprints off his back, using them to construct the outer hull of the robot. With the robot partially complete, SHADOW tested out its power by having it destroy a city bloc. They later captured Akira and were able to complete the robot, but Kikaider and 01 arrived in SHADOW's base to rescue Akira and destroy Giant Devil. Giant Devil activated to fight the androids, but the androids were able to destroy it by using a combined attack on the Giant Devil's weak spot, its head. With the destruction of Giant Devil, SHADOW's base was also blown up and the blueprints for Giant Devil were lost as well.