Petronella Osgood

Osgood was a scientist who worked for UNIT. During the attempted Zygon takeover of UNIT HQ in the early 21st century, she acted in a capacity of a personal assistant to Kate Stewart.

She was present when the Eleventh Doctor's TARDIS, along with 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 long multi-coloured scarf, which was strikingly similar to one he wore in his fourth incarnation. She displayed 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 fellow scientist McGillop analysed the stone dust in the undergallery under the National Gallery, and it Osgood who realised the dust was the smashed remains of the statues that had previously occupied. She then realised with horror that their places had been taken over by the Zygons that had escaped from the paintings. She was able to escape from them, although she was duplicated by one of their number before she got away.

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, annoyed at "getting one with a defect," and promptly took her inhaler. When she escaped from this Zygon by tripping it with her scarf, she reclaimed her inhaler.

Osgood was in the Black Archive with her Zygon version when Kate threatened to destroy London, rather than let the alien technology in the archive fall into Zygon hands. Like the other humans and Zygons in the room, her memory was temporarily cleared by three incarnations of the Doctor, so that she did not recall whether she was the real Osgood or her own Zygon duplicate. As peace talks ensued, she realised who the real Osgood was when the Zygon duplicate began wheezing and did not have the inhaler. She gave it to the Zygon version, and both Osgoods agreed to keep their discovery a secret so as not to disrupt the peace talks.

Osgood had a sister to whom she negatively compared herself. (TV: The Day of the Doctor)

Behind the scenes
On 24 July 2014, Ingrid Oliver appeared in character as Osgood in a video thanking fans for helping the official Doctor Who Twitter account reach one million followers.