A towering android left behind by the Old Ones of Exo III, who served Roger Korby until he remembered the fate of his creators.
Ruk is a huge android built by the long-vanished inhabitants of Exo III, who had abandoned their organic bodies for machines. Discovered by Roger Korby, he obeys Korby's orders and kills a member of Kirk's security detail by throwing him into a chasm.
Kirk prompts Ruk to recall that the androids once destroyed their creators to ensure their own survival. Realising that Korby poses a similar threat, Ruk turns against him and is destroyed by Korby's phaser. He was played by Ted Cassidy.