General Kellen was a high ranking military officer within the Klingon Empire who was infamous in the Federation due to his role in the Muscari Incident. He was highly regarded among other races for his reputation for remaining cool-headed in a fight in contrast to the typical response of other Klingons at this time to simply dive in. He was in command of a task force of ships which he led from the IKS Qul.

He was involved in the first encounter with the alien species known as the Furies, "recognising" them as the Klingon Havoc, who were said to consume failed warriors. After his fleet narrowly survived the Rath's first appearance in the Alpha Quadrant, Kellen travelled to the nearby planet of Capella IV to make contact with Captain James T. Kirk, who was helping the Capellans defend themselves against a Klingon occupation, arranging a cease-fire of the conflict that was being waged between Klingons and Federation forces at that time to ask for Kirk's help in fighting the Furies. Although initially dissatisfied with Kirk's decision to try and talk to the demons, his thoughts on Kirk improved after Kirk coordinated the Klingon forces in a successful attack on the Rath, agreeing to stand trial for his murder of a Starfleet ensign when he tried to attack the Furies himself. As the two parted, Kellen noted that Kirk's decision not to act rashly would make him an interesting opponent, noting that a war between their two cultures would be glorious. (TOS - Invasion! novel: First Strike)