Dr. Elizabeth Klein was an inhabitant of a defunct alternative timeline accidentally created when the Seventh Doctor and Ace left a piece of laser technology in the hands of the Nazis, leading to their victory in World War II. She was the sole remnant of that timeline and later a companion-in-adversity of the Seventh Doctor.
Biography
Klein was a favourite of the Fuhrer, assigned many duties on behalf of the Reich. In the early 1950s of her timeline, she was commissioned to investigate the outbreak of a deadly virus in Kenya. She failed to trace its source, and was called off the mission before she could investigate further. This failure would continue to haunt her as the years went by.
In the 1965 of her timeline, Klein was working in the physics department at Cambridge when she was recruited by Jonas Faber to analyse alien technology in the possession of the Reich. She became romantically involved with Faber, who eventually gave her access to a device which had previously been kept locked away from everyone – the TARDIS. She was told its pilot had been fatally shot by a border patrol in 1944, after which his body had been stolen. Klein could not enter the vessel until a mysterious man calling himself “Johann Schmidt” provided her with the key. In reality, Schmidt was an alternate version of the Doctor’s eighth incarnation who, claiming to be a fellow scientist, fabricated a “travel log” for the TARDIS which would allow Klein to pilot the ship back to any of its recent destinations. Schmidt manipulated her into deciding to travel back to 1944 to retrieve the Doctor to make him teach her how to pilot the TARDIS. Faber eventually saw through Schmidt’s trickery, but not before Klein had already decided to disobey his orders and take the trip into the past. She left her lover behind, promising to make him Fuhrer through her efforts. (BFA: Klein's Story)
Unfortunately for Klein, her arrival in 1944 alerted the Doctor to the impending change in history, and he prevented it, stranding her in the prime reality. Klein, believing her mistake had destroyed what she considered the correct version of history, vowed to one day get the TARDIS back and restore her world. (BFA: Colditz)
Along with other Nazis who fled the Allied invasion of Europe and the fall of Berlin, Klein ended up in South America, where she became associated with Hans De Flores. There she got a medical doctorate in addition to her doctorate in physics. Afterwards, she set out for Africa, where she hoped that the events of her timeline would repeat themselves in the 1950's of our universe, giving her another chance to discover the source of the mysterious virus. Isolated in a house with several other women during the Mau-Mau rebellion, she again met the Seventh Doctor. Despite their animosity, they worked together to solve the mystery of the virus and stop an alien plot to use Earth as a testing ground for biological warfare. The Doctor, knowing Klein would continue to pose a threat to history, insisted she accompany him as a companion on his travels so that he could keep an eye on her. (BFA: A Thousand Tiny Wings)
After Klein and the Doctor left Kenya, he asked her to tell him the story of how she came to pilot the TARDIS to 1944 in the first place. Upon hearing the details, he explained to her that the enigmatic “Johann Schmidt” had, in fact, been an alternate future regeneration of himself. Klein was furious to learn her world had been lost solely through the Doctor’s manipulation of her, but pretended to accept his viewpoint so she would have a chance to turn the tables on him. She travelled with him for a lengthy time, during which she learned how to control the TARDIS by watching him. Her chance to betray the Doctor came when he lost his TARDIS key while trying to help a Vrill warrior hatch from its egg. Klein recovered the key and abandoned the Doctor on the Vrill planet, taking off in the TARDIS to restore her own timeline. (BFA: Survival of the Fittest)
Klein tried to use the TARDIS to restore her exact timeline, but was unsuccessful in the end. She instead created what she described as a far better one. The details of the changes she made to history are unknown, except that the new Reich began in 1944. She avoided trying to bring back her lost love, Jonas Faber, because she inwardly knew she would never be able to perfectly restore the past they had lost. By 2044, Klein had used time travel to engineer the defeat of numerous formidable alien threats, including the Sontarans and even the Daleks. However, although the Doctor’s past had been completely rewritten, the memories of his experiences with Klein resurfaced in 2044, replacing those of his alternative self. Before this, he had arranged for an attack by Selachians from the future on Earth's moonbase to lure Klein and the TARDIS there. The Selachians used future technology to destroy the TARDIS, separating its internal and external dimensions and stranding Klein in the middle of the invasion before she could prevent it by equipping the past with knowledge from the future. The Doctor told Klein that she had been tried (presumably by the Time Lords, though he did not mention them by name) and took her out of the timeline with another TARDIS specially equipped to carry out her sentence. She willingly submitted to her punishment – total obliteration from time – in the hope that it might restore her original timeline by preventing her from ever meeting the Doctor. The actual outcome was quite different. The prime reality was restored and another Dr. Elizabeth Klein came to be: a Klein known for her triumphs working for UNIT. Her personality appeared to be the same, except that this new Klein devoted her talents to the protection of Earth. Upon meeting the Doctor in 1962, she did recognise him slightly despite the fact that her experiences with him had been erased. (BFA: The Architects of History)
Personality
Klein is a fervent believer in National Socialism, and the benefits to society of fascism in general. Despite holding the view that the needs of the many outweigh the needs of the few, she formed close, compassionate relationships with the people in her care, as evidenced by the impact Lucy's death had on her (BFA: A Thousand Tiny Wings), and her concern for the Vrill (BFA: Survival of the Fittest). Despite knowing the atrocities which Nazis like Mengele committed, she differentiated their actions and their ideals, saying that Mengele went too far. Still, she was not beyond coolly executing an enemy or torturing them for information. She was not intimidated by the Doctor or his reputation. She was not a hypocrite; she was willing to die for the greater number of people to survive, although she admitted that she would prefer to live. Rather like Liz Shaw, Klein was recruited from university, and showed an interest in and knowledge of the more esoteric and experimental aspects of science, such as time travel. She was described by Faber as being unlike many of her contemporaries, having an open, questioning, mind. Before her removal from history, she came to agree with the Doctor on some points, particularly with his observation that no matter how much she changed history, it would never feel completely "right".