Kayn was the main antagonist of the Blake's 7 episode "Breakdown".
Kayn was a neurosurgeon working at the experimental research station XK72. Even though the station was meant to be politically neutral, and even though he was irritated by the bureaucracy of the station administrator Farren, Kayn was a supporter of the Terran Federation, seeing them as bringing order to the galaxy.
When the Liberator arrived at the station, seeking a neurosurgeon to assist them in repairing Gan's limiter, Kayn immediately recognised Blake and his crew, who had claimed to be the civilian crew of an experimental Federation ship. He teleported over to the Liberator but, to the disgust of his assistant Renor, he not only sent a message to the Federation informing them where the Liberator was, but delayed the operation in order to keep the Liberator there. He approved of the use of limiters, believing they controlled dangerous psychopaths.
Both Avon and Vila realised what Kayn was doing and confronted him. Kayn agreed to carry out the operation but told them the Liberator would have to remain stationary. Blake told him that if he didn't complete the operation before the Federation arrived, he would destroy his hands. Kayn did as requested and was teleported back to XK72. As the Liberator and the Federation ships faced off nearby, Kayn tried to order XK72 to fire on the Liberator. Farren objected and Kayn beat him to death, an act that shocked even him. Minutes later, Kayn was killed when the Federation ships accidentally destroyed XK72 when the Liberator dodged their fire.