Petronella Osgood

Osgood was a scientist who worked for UNIT when Kate Stewart was in charge, and during the attempted Zygon takeover of UNIT HQ in 2013, acted in a capacity of a personal assistant to Kate.

She was present when the Eleventh Doctor's TARDIS, alongside both the Doctor and Clara Oswald, were brought to HQ in an effort to understand what had happened to several paintings in the Under-Gallery. The Doctor took a liking to her large scarf, which was strikingly similar to one he wore in his fourth incarnation. She also had a type of hero-worship belief in the Doctor, twice "praying" to him to save her during moments of danger.

On the Doctor's orders, she and a fellow scientist analysed the stone dust in the undergallery under the National Gallery, and it was her that realized the dust was the result of the statues that had previously occupied the room being smashed. She also realized with horror that their places had been taken over by the Zygons that had escaped from the paintings, but was able to escape from them, although she was duplicated by one of their number before she got away from them. When in the Black Archive with her Zygon version, she realized who the real Osgood was because the real Osgood had the inhaler and gave it to the Zygon version. The real Osgood's and Zygon Osgood's memory had been cleared and they didn't know if they were the real versions or the Zygon versions. However, both Osgood's agreed to keep their discovery a secret so as not to disrupt the peace talks.

She was asthmatic, and often forgot to use her inhaler when she needed to, especially when over-excited, often being reminded by Kate when she started wheezing. The Zygon who took her form disliked this and was annoyed at "getting one with a defect," and promptly took her inhaler.

It was revealed as a Zygon took on her form that she had a sister to whom Osgood negatively compared herself. (TV: The Day of the Doctor)