Lady Peinforte was a cavalier of Windsor in 1638, as well as a sorceress and poisoner. She opposed the Seventh Doctor.
Biography
She was an expert archer who fought in battle against the roundheads in the name of King Charles I. In 1638 when a large piece of validium fell to Earth, Lady Peinforte found it, sculpted it into an image of herself holding a bow and arrow and called the result "Nemesis". The Doctor found the statue, removed the bow and arrow, and placing it onto the rocket-sled, fired it into space. Before that, however, the statue told Lady Peinforte of Gallifrey, the Time Lords, the Dark Times and the history of the Doctor.
Seeking to retrieve the statue, she hired a mathematician, whom she soon poisoned, to calculate the date of the Nemesis' return to Earth. With her servant, Richard Maynarde, and black magic, they travelled forward in time to Windsor in 1988, claiming as they departed, "We ride to destiny". (TV: Silver Nemesis)
The Seventh Doctor later said that Fenric, rather than Peinforte alone, had enabled her to travel through time. (TV: The Curse of Fenric)
When the comet landed, she and Richard stayed hidden, observing local police, De Flores' Neo-Nazis, and the Cybermen. During a skirmish between the Neo-Nazis and Cybermen, she struck two Cybermen with poison-tipped arrows, convinced her poison was perfect, when it was the gold heads of the arrows that really killed them.
Lady Peinforte and her servant made their way to her tomb, where the Cybermen had hidden the statue, believing that foreknowledge of her death would drive her mad.
Finally, she made her way to where the Seventh Doctor had hidden the rocket-sled and called the Nemesis to it. Lady Peinforte threatened to reveal the Doctor's secrets if he would not give her the Nemesis bow. He refused and gave it to the Cyber-Leader. The Cyber-Leader expressed no interest in knowing the secrets Lady Peinforte offered to tell him, driving her mad. Just before the Nemesis prepared to lift off from Earth as the Doctor had programmed it, Lady Peinforte jumped onto the statue and merged with it. In space, the Nemesis exploded, destroying the entire Cyber-Fleet and Lady Peinforte as well. (TV: Silver Nemesis)
Peinforte's power was a remnant of the Pythia's. (PROSE: Cat's Cradle: Time's Crucible)
Behind the scenes
According to The Doctor: His Lives and Times, she was the widow of Sir Roger Peinforte. She inherited his estate after his "strange and sudden death".