Petronella Osgood was a scientist who worked for UNIT, acting in a capacity of a personal assistant to Kate Stewart, in the early 21st century. Osgood was an admirer of The Doctor and had read all of the UNIT files regarding every one of his incarnations. She finally had a chance to meet her icon when Zygons attempted an invasion of Earth. Not only did she meet the Eleventh Doctor, but she had a chance to meet the War Doctor and Tenth Doctor, as well (The Day of the Doctor). The War Doctor is one that she hadn’t read about, as The Doctor preferred to keep his existence secret. At that time, he stopped the Zygon invasion and instigated a ceasefire and treaty between the Zygons and Humans. The next time she would meet The Doctor would be when Missy, the latest incarnation of The Master, created an army of Cybermen form the planet’s dead to take over (Death in Heaven). During this event, Osgood was vaporized by Missy. What Missy didn’t know was that after the treaty with the Zygons was made, the Zygon who had taken Osgood’s form kept that form. The Osgood “sisters” had become the embodiment of the treaty, refusing to acknowledge which was the Human and which was the Zygon. (The Zygon Invasion). After the rebellion of a splinter group of Zygons, their leader, Bonnie, would take up the identity of the second Osgood. They said that one day they would say which was Human and which was Zygon when the answer wouldn’t matter anymore(The Zygon Inversion)
Osgood was a fanatic admirer of the Doctor. She dressed according to his fashion senses from various incarnations, choosing to wear a similar scarf to that worn by the Fourth Doctor (The Day of the Doctor). She later wore a leather jacket similar to the one worn by the War Doctor, trainers similar to those worn by the Tenth Doctor and a bow-tie similar to that worn by the Eleventh Doctor. She also admitted that “bow-ties are cool” – a phrase often used by the Eleventh Doctor. She also wore a shirt with question marks on the collars, like the Fourth, Fifth and Sixth Doctors, and a cardigan similar to the Seventh Doctor’s (The Zygon Invasion). 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 (Death in Heaven).
Osgood was also very intelligent. She alone was able to work out the reason why the Zygons smashed the statues in the Under-Gallery. (The Day of the Doctor) She also surprised the Doctor by guessing correctly that Missy was the Master regenerated into female form: UNIT kept files on all past Prime Ministers, including the Master. (Death in Heaven) She also could come up with a plan for killing the Doctor if she was evil, by shooting him between the eyes for each regeneration, never allowing him to talk. (The Zygon Inversion)
She managed to figure out whether she or her Zygon doppelganger was the real Osgood after the Tenth and Eleventh Doctors jumbled their minds, making them unable to determine their own species, in an attempt to facilitate negotiation of the ultimate peace treaty. She managed to determine this as her Zygon counterpart suffered an asthma attack, motivating her to pass her asthma inhaler to the other Osgood. (The Day of the Doctor)
Osgood had a sister to whom she negatively compared herself. (The Day of the Doctor) Missy understood that she had little confidence in herself. Osgood was able to keep her cool, even when faced with the threat of death from Missy and tried to reason her way out. (Death in Heaven)