Klag's father was captured by the Romulans in battle and was not allowed die an honorable death. He had later escaped and was living without honor on Qo'noS, where he awaited death. Klag was ashamed of his father, and had not talked to him since his return.
Later in 2365, Klag was suspicious of his new first officer, Commander William T. Riker, assigned to him as part of an Officer Exchange Program between the two governments. He challenged Riker's authority, so, following the Klingon way, Riker fought back. This gained him the respect of Klag, and later they became friendly, talking with each other in the mess hall. When Captain Kargan blamed the USS Enterprise-D for planting subatomic bacteria that were eating away at the hull, Klag stood up for Riker. He said that Riker "may be a spy, but he is no coward." After Riker had taken control of the Pagh, Klag followed his orders, and when Kargan returned, Riker allowed Kargan to strike him (discipline him, Klingon fashion) in order that Kargan might regain the respect of the crew. As Klag helped him up, he commented that Riker understood Klingons better than he thought. (TNG: "A Matter Of Honor")
Klag was played by recurring Star Trek actor Brian Thompson in his first appearance in the franchise. He would later portray an unnamed Klingon helm officer serving aboard the ill-fated Duras sisters' Bird-of-Prey in Star Trek Generations.
His mirror universe counterpart (β) is depicted in the short story "Family Matters" contained in the anthology Shards and Shadows and the novels Warpath and Rise Like Lions. In the latter, he succeeded Martok as Regent of the Klingon-Cardassian Alliance in 2377 after killing him in honorable combat. He was himself succeeded by Duras in 2378.
In the backstory to Star Trek Online Klag had become a general by 2403, and led the Klingon invasion of the Gorn homeworld that ended a war that had been raging off and on since the 2380s. He appears in the game proper as a non-player character at Ganalda Station.