|Vehicles and/or Gadgets||None|
|Occupation||Leader of U.N.I.T.|
Kate Stewart, also known as Kate Lethbridge-Stewart and nicknamed "Tiger" by her father, was the Chief Scientific Officer of UNIT. She was the daughter of Alistair Gordon Lethbridge-Stewart and his first wife Fiona, and the mother of Gordon Lethbridge-Stewart and another child.
Kate spent her youth not knowing of her father's activities with extra-terrestrial beings, believing whole-heartedly that his life consisted of a boring and simple military job. She and her mother spent much time ironically joking that he was constantly off on amazing adventures when they both knew the "truth." Kate would not begin to scratch the true surface of her father's life until years later when she shared an encounter against the Great Intelligence with him. She would spend some time investigating UNIT and her father's ties to it, at one point encountering ex-UNIT operative Douglas Cavendish to investigate a "haunting," which revealed itself to be a Dæmon. She soon joined the organization, rising to its top position and changing it to be a more scientific, peaceful group, though not afraid to resort to more violent solutions when pressed.
As the leader of UNIT, Kate encountered the Doctor many times during encounters between Earth and alien threats, becoming as familiar to the Doctor as her father had been. She helped protect the Earth on several occasions, leading with Cybermen, the new female Master and an invasion of Autons via the company Delvin Futuretech. She was also at the forefront of the Human-Zygon coalition, setting a new precedent for peace not previously seen in the organization against the Silurian threat. This peace was challenged by a group of radical Zygons and almost led to Kate and the lead Zygon using the Osgood Box as a means of wiping each other out, but the Twelfth Doctor was able to convince both of the futility and childishness of conflict. In fact, neither Box actually functioned but instead acted as a catalyst for change, whereby the Doctor could wipe the user's memories using the effects of the UNIT Storage Facility.