Petronella Osgood

Petronella Osgood was a human who worked as UNIT's scientific advisor under Kate Stewart. She 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.
== Biography ==
==== Early Life ====
When Osgood was young, she was jealous of her sister, who she thought was prettier (TV: The Day of the Doctor) Later, Osgood became UNIT's scientific advisor and wore the Doctor's scarf that she found in the Grey Archive (TV: The Day of the Doctor) She and/or her Zygon duplicates would later dress like the Seventh Doctor as well. (TV: The Zygon Invasion)
==== Operation Double ====
After the Eleventh Doctor called Kate from the TARDIS, Osgood excitedly joined her in meeting with him and, at his request, analysed the stone dust in the Under Gallery. She and McGillop realised that Zygons had smashed statues to enable them to hide in plain site and were attacked and duplicated, although Osgood managed to escape. In the Black Archive, the War, Tenth and Eleventh Doctors made Osgood, Kate and the Zygons forget who were humans and who were Zygons in order to allow them to create the perfect peace treaty. (TV: The Day of the Doctor)
Following the ratification of Operation Double and the Doctor handing over the Osgood Boxes, (TV: The Zygon Invasion) the Doctors restored everybody's memories, although this left the Zygons unconscious for a time. Osgood wrote her double a letter in which she invited her to continue using her form. (PROSE: The Day of the Doctor) The two Osgoods administered and embodied the peace between their two races and kept an eye on the Osgood Boxes, refusing to confirm which of them was human and which was Zygon. Together, they recorded an explanation of Operation Double (TV: The Zygon Invasion) as well as a message seeking to dissuade anybody who should capture them from searching for the boxes. (TV: The Zygon Inversion)
=== Nightmare Scenario ===
After Osgood's Zygon double was killed by Missy, (TV: Death in Heaven) she went mad with grief and eventually went undercover to the United States. She visited Truth or Consequences where fights broke out between the humans and Zygons and informed the Doctor of the Nightmare Scenario when she was attacked by the Zygon rebels. She was captured and taken to Turmezistan where she was interrogated for information on the Zygons' whereabouts and to give UNIT the rebels' demands.
The Twelfth Doctor saved Osgood but their aeroplane was shot down by Bonnie, (TV: The Zygon Invasion) after which they took a van to South London to find her and realised after meeting Etoine that she planned on unmasking the Zygons. They saved Clara Oswald and, after the Doctor talked Bonnie out of her revolution, Bonnie took on Osgood's form to help her keep the peace. The Doctor offered to take Osgood in the TARDIS, but she turned the offer down due to her responsibilities and returned to work with her new sister. (TV: The Zygon Inversion)
== Personality ==
Osgood was a "fan" of the Doctor and knew of many of his previous incarnations. She would pick iconic items (i.e. multi-coloured scarf, bow tie) from the incarnations she knew of and wore them herself. (TV: The Zygon Inversion) When asked by the Twelfth Doctor if she would like to travel with him, she was so excited that she had to use her inhaler. (TV: Death in Heaven)
Osgood was very intelligent. She alone was able to work out the reason why the Zygons smashed the statues in the Under Gallery. (TV: The Day of the Doctor) She also surprised the Twelfth Doctor by correctly presuming that Missy was the Master regenerated into female form: UNIT kept files on all past Prime Ministers, including the Master. (TV: Death in Heaven) She also came up with a hypothetical plan for invading Earth by killing the Doctor by shooting him between the eyes for each regeneration, never allowing him to talk. (TV: The Zygon Inversion)
She managed to figure out whether she or her Zygon doppelganger was the real Osgood after the Tenth and Eleventh Doctors, joined by the War Doctor, jumbled their minds, making them unable to determine their own species, in an attempt to facilitate negotiation of the ultimate peace treaty. She discovered this when her Zygon counterpart suffered an asthma attack, motivating her to pass her asthma inhaler to the other Osgood. (TV: The Day of the Doctor)
Osgood had a sister to whom she negatively compared herself. Missy observed that she had little confidence in herself. (TV: Death in Heaven) Osgood proved capable of such courage, and was willing to fight back and go to the rescue. (TV: The Day of the Doctor, The Zygon Invasion|The Zygon Inversion)