"More complex than Brown. Much superior. I was left here by the Old Ones."
Ruk was tall, broad and immensely strong, capable of lifting a Human male easily with one hand. Despite his size, he could move silently and gracefully, making him an extremely dangerous physical opponent. He could duplicate voices effortlessly – his actual voice was very low-pitched and gravelly.
He was discovered in the underground ruins of that planet in 2261, by Dr. Roger Korby, where he was still tending the machinery the Old Ones had left behind. Even he did not remember how many centuries he had been doing this work. Ruk helped Korby learn how to use the ancient machinery, including the android duplicator. With Ruk's help, Korby built other androids, including Brown and Andrea.
In 2266, Captain James T. Kirk and Christine Chapel arrived at Exo III in search of Korby. By then, Korby had reprogrammed Ruk to serve him, and had devised a scheme to infiltrate android duplicates into society slowly and carefully. As part of that scheme, Korby and Ruk imprisoned Kirk and Chapel, and built a duplicate of Kirk to take control of the USS Enterprise.
Questioned about his origins by Kirk, Ruk remembered that the androids, presumably including himself, had become frustrated by the illogic and inferiority of the Old Ones. In turn, the Old Ones became frightened of their creations, and began turning them off. Their survival at stake, the androids overcame their programming and slew their makers. Once Ruk remembered this, he overcame Korby's programming the same way. No longer able to control Ruk, Korby destroyed him. (TOS: "What Are Little Girls Made Of?")
In the revised final draft script of "What Are Little Girls Made Of?", Ruk was characterized as a "huge, enormously tall, completely hairless half-Human creature." He was also scripted to have a "heavy" voice. 
The book Star Trek: The Original Series 365 (p. 066) refers to Ruk as "a monster right out of the classic novel Frankenstein." Ruk's facial appearance involved makeup designed by TOS Makeup Artist Fred Phillips. He emphasized the natural planes of Ted Cassidy's face with grease pencil and covered the actor's hair with a latex bald cap. The costume Cassidy wore as Ruk was designed by TOS Costume Designer William Ware Theiss. (The Star Trek Compendium, 4th ed., p. 41)
According to the novella "The Worst of Both Worlds", the mirror universe James T. Kirk of the ISS Enterprise destroyed the last surviving member of the android society (presumably Ruk's mirror counterpart) on Exo III, who had likewise turned on their masters and killed them.