Lethe was a patient who was committed to the Tantalus Penal Colony in the mid-23rd century. By 2266, Lethe had been completely rehabilitated by Doctor Tristan Adams and his staff, and elected to remain behind on the colony as a therapist.
She accompanied Dr. Adams in welcoming Captain James T. Kirk and Dr. Helen Noel to the colony when the USS Enterprise crewmembers came to investigate the facility. When questioned about the offense she had committed to be placed in the facility, she replied that it didn't matter and remarked that her "previous self" was a malignant and hateful person. (TOS episode & novelization: Dagger of the Mind)