Kor is an antagonist turned ally in the Star Trek franchise. He is the franchise's first Klingon antagonist, having been the main antagonist of "Errand of Mercy", the episode which introduced the Klingons. He later appeared in one episode of the animated series and three of Deep Space 9.
In his live action appearances, he was played by the late John Colicos. In his animated appearance, he was voiced by the late James Doohan.
At the outbreak of war with the Federation in 2267, Kor was assigned the task of conquering the strategically important planet Organia. He occupied the planet without resistance, naming himself its military governor, and introduced several restrictive laws, such as curfews and the banning of large gatherings. He also took hostages, threatening them with execution in the event of the laws being disobeyed. Unknown to him, Kirk and Spock were present on the planet and, when they destroyed a munitions dump, Kor ordered the executions to be carried out. Kirk and Spock attacked his headquarters while the Federation and Klingon fleets faced off in orbit, but suddenly they found weapons and their opponents' bodies too hot to touch. It was discovered that the Organians, assumed to be a medieval society, were actually energy beings who only appeared humanoid, and demanded the war end. They told Kirk and Kor that one day humans and Klingons would be friends, a prospect Kor in particular found horrifying.
Kor would encounter Kirk again a few years later, when he ambushed the Enterprise only for both ships to be drawn into a time trap. Other ships had been trapped there for centuries, building a society, and urged the newcomers to join them. Kirk and Kor worked out that both ships working together would be able to escape the time trap but Kor had no intention of letting the Enterprise leave and had the ship sabotaged so it would explode shortly after leaving. Fortunately for the crew, they discovered the sabotage in time but after Kor had departed.
At some point, Kor joined with fellow Klingon captains Koloth and Kang, together with the Federation ambassador Curzon Dax, to defeat the forces of a space pirate, the Albino. However, the Albino responded by infecting the three Klingons' firstborn sons with a virus that killed them. The four men swore a blood oath to kill the Albino. In 2370, Kor and Koloth were summoned to Deep Space 9, where Curzon's new host Jadzia Dax was stationed, by Kang, who had located the Albino. In fact, Kang was planning a suicide attack, since he knew the Albino was expecting them. Dax persuaded him to even the odds by disabling the weapons of the Albino's guards, giving them the edge in hand-to-hand combat. Kor and Koloth fought off the Albino's guards, although Koloth was killed in the process, while Kang and the Albino killed each other, leaving Kor as the only one of the Klingons to survive.
Kor returned to the station two years later to ask the help of Dax and Worf in locating the fabled Sword of Kahless, which he had learned was located in the Gamma Quadrant. He dreamed of being the one to return it to Qo'nos and present it to the Emperor. They found the sword but had to contend with the efforts of Toral, Worf's old enemy, who also sought it. As the trio fled through the tunnels, Kor began to dream of not just returning the sword home but of using it as a symbol to rule the Empire. Worf began to develop similar ambitions and the two Klingons became increasingly irrational, treating each other as enemies. It took another battle with Toral and his men, and them both being stunned by Dax, to bring them to their senses. Realising that any other Klingons who encountered the sword would develop the same dreams of grandeur, they chose to cast it adrift in space.
When the Klingons joined the Federation in war against the Dominion, Kor sought to join the effort but found he had two many political enemies. He went to Worf for help. General Martok was unwilling to give Kor a command, since years earlier, Kor had blocked Martok's application to be an officer on the ground he was not from a noble house. Worf however made Kor third officer of Martok's flagship. Although he was initially admired by the younger officers, it became clear Kor's mental faculties were failing him: During an attack on a Dominion outpost, he became confused, believing he was back in an old battle against the Federation, and was ridiculed by Martok and the others. Martok's aide gave Kor the chance to redeem himself: The Klingon taskforce was being pursued by a larger and faster group of Jem'Hadar ships and needed someone to sacrifice themselves. Kor arranged to take command of one of the ships with a skeleton crew, fighting a solo rearguard action while the rest escaped and ensuring he would be remembered as a hero.
- There were a number of attempts to bring Kor back during the original series which fell through because of John Colicos' other commitments. Ironically, he was replaced by the characters of Koloth and Kang, who would later appear with Kor in the episode "Blood Oath".