An alien explorer on Pyris VII who takes the form of a woman and a black cat and becomes intoxicated by human sensations in Catspaw.
Sylvia arrives on Pyris VII with Korob, and like him she has assumed a humanoid form to study the Federation. Using the transmuter, she can transform into a black cat, cast illusions of witchcraft and take over minds, as she does with Scott and Sulu.
In Catspaw, she becomes enthralled by human sensations and Kirk's attentions, and turns on Korob when he wavers. When Kirk destroys the transmuter, she and Korob revert to their true, tiny forms, and they die in the planet's atmosphere. She was played by Antoinette Bower.