He and his fellow androids eventually rebelled against the Old Ones when they realized that they were superior to their creators. The Old Ones began to deactivate the androids but they could not avert their own extermination as the androids overcame their programming and slew their masters. (TOS episode & Star Trek 11 novelization: What Are Little Girls Made Of?)
Whilst their creators were dead, the androids developed a degree of paranoia toward all organic life, who they believed were a threat to their existence. As the Old Ones never developed space flight technology, the androids were restricted to Exo III and hoped that another intelligent, organic species would discover them. When such an event occurred, the androids planned to take over the invaders' space craft and use it to escape in order to find a solution to their inherent problem that the Old Ones denied them. Until that time, the androids decided to enter into stasis and left one of their own, Ruk, behind to serve as caretaker as well as watchman, while they were slumbering. Ruk tended to the machines left behind by the Old Ones throughout the intervening millennia. (TNG novel: Immortal Coil)
Ruk was discovered by Doctors Roger Korby and Aaron Brown in 2261. Korby eventually managed to reprogram Ruk, who assisted him in the creation of four additional androids: Andrea, a duplicate of Korby himself and two of Brown after the deaths of their Human bodies. (TOS episode & Star Trek 11 novelization: What Are Little Girls Made Of?, TOS novel: Double, Double)
By 2266, Korby had developed a plan to slowly infiltrate android duplicates into society with the end goal of replacing all humanoid lifeforms in the galaxy with androids. When Captain James T. Kirk and Nurse Christine Chapel of the USS Enterprise arrived on Exo III in that year, Ruk imprisoned them on Korby's order and assisted him in the creation of an android duplicate of Kirk. Kirk was able to convince Ruk that his survival was threatened by Korby by reminding him of the rebellion against the Old Ones which had occurred hundreds of thousands of years earlier. Ruk turned against his new master but was almost immediately vaporized by Korby.
Ruk described himself as "more complex than Brown. Much superior." He was considerably taller, larger and stronger than most humanoids, being able to effortlessly lift Kirk with one arm. He also had the ability to mimic voices perfectly. (TOS episode & Star Trek 11 novelization: What Are Little Girls Made Of?)
In an alternate reality, Ruk was a member of the Imperial Elite, the super-powered enforcers of the Imperial Planets Science Police. In this reality, Ruk had been reconstructed with one-half of his body built with different technology. With his robotic brain, Ruk was able to control fellow Elite member Validus—a similar arrangement to the way an alternate Validus was enslaved by the cyborg Tharok in the reality of the Legion of Super-Heroes. When encountering Ruk, Captain James T. Kirk recognized the android after having encountered him in the primary universe.
When the Legion and a landing party from the Federation starship USS Enterprise were trapped in the Imperial Planets' timeline, Ruk and the Imperial Elite squared off against the time-travellers and were defeated. Ruk in particular was defeated when Captain Kirk tried to lunge the android, allowing himself to be grabbed into a stranglehold—much as he was in his first battle with the other Ruk—giving Cosmic Boy a chance to immobilize Ruk with magnetic power. (TOS - Legion of Super-Heroes comic: "Issue 3")