- You may be looking for the Petra Williams of the main universe.
In a parallel universe, Doctor Petra Williams was the second in command of the Inferno Project at the the Eastchester Scientific Labour Camp. Unlike her counterpart in the Doctor's universe, whose hair hung loose, the alternate Petra wore her hair drawn up in a bun.
Dr Williams was loyal to her employers until the true danger was revealed, after which she assisted the Third Doctor to get back to his own world. She was last seen hugging Greg Sutton, with whom she had formed a relationship, as the lava from the shaft poured onto the Earth's surface. (TV: Inferno)