Karl was a male Human inmate at the rehab colony on Tantalus V in the 23rd century.
In 2266, as he was walking with a female inmate in a corridor, Dr. Tristan Adams greeted Karl. At the time, the doctor was giving Captain James Kirk and Dr. Helen Noel a tour of the colony. Karl had the same blank look as the other staff members and inmates there; they had been victims of the neural neutralizer. (TOS: "Dagger of the Mind")
Karl was portrayed by an unknown actor.