Petronella Osgood

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Petronella Osgood was a human who worked as UNIT's scientific advisor under Kate Stewart. Osgood proved invaluable in thwarting a number of alien threats to Earth and, with a Zygon double who was replaced by Bonnie after she was killed by Missy, became one half of the living embodiment of Operation Double. She was dedicated to ensuring peace between humanity and the Zygons to the point that she declined the offer of travelling with her hero, the Doctor. The Osgoods helped fight alien threats, keeping secret which of them was which, and one briefly served as Chief Scientific Officer after Kate's disappearance.