Sylvia was the junior member of a team from beyond the Milky Way Galaxy of two Ornithoid explorers in Human form who established an outpost, disguised as a large castle, on the planet Pyris VII. Landing parties from the USS Enterprise were trapped by Sylvia and her colleague Korob for observation in 2267.
In the form of a black cat In the place where these aliens came from, there was evidently no luxury and little sensation. Sylvia's sudden immersion in sensation and luxury caused her to become irrational; she discovered, too, that she liked inflicting pain and controlling others. Korob considered Sylvia a traitor and her actions a betrayal of the purpose for which the Old Ones dispatched the explorers. But by that time, Sylvia was past caring; in fact, she planned to dispose of Korob and seize his greater power for her own use.