K'Kath is a holographic Klingon youth, one of the two rough friends who lead the Doctor's simulated son Jeffrey astray after Torres rewrites the family programme in Real Life.
K'Kath appears in the third-season episode Real Life. The Doctor's first holographic family is so sugary that B'Elanna Torres adds some randomised realism, and one result is that his teenage son Jeffrey starts keeping company with two Klingon boys, Larg and K'Kath.
The pair lounge about the house, play loud Klingon music and treat the Doctor with open contempt. Worse, they are preparing Jeffrey for a ritual of passage involving a blade and the spilling of blood, which he regards as a far more interesting heritage than his own.
The Doctor's attempt to forbid the friendship only drives Jeffrey further away, one of several domestic crises the programme throws at him before the accident to his daughter Belle teaches him what family life can cost.