Petronella Osgood

Osgood was a scientist who worked for UNIT, acting in a capacity of a personal assistant to Kate Stewart, in the early 21st century.

An admirer of the Doctor, she met the Time Lord on two occasions, first was the infiltration of UNIT by Zygon invaders which was resolved by the Eleventh Doctor joined by the Tenth and War Doctors (TV: The Day of the Doctor) and the last being with the Twelfth Doctor during an invasion of Earth by an army of Cybermen led by a new, female incarnation of, at whose hands Osgood was murdered. (TV: Death in Heaven)

Meeting the Doctor
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 was 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. (TV: The Day of the Doctor)

Missy and murder
After began to deploy her Cyberman army across the world, Osgood confronted and secured her. She was stationed near Missy during her captivity aboard Boat One, who told Osgood: "I'm going to kill you in a minute," calling it their "secret girl plan". Missy preceded to countdown to Osgood's death, tormenting her after each number. She then escaped her handcuffs, killed her guards and grabbed hold of Osgood. Despite claiming to be more useful to Missy alive, Osgood was vaporised, with only her glasses remaining intact until Missy crushed them under her heel. (TV: Death in Heaven)

Personality
Osgood was a fanatic admirer of the Doctor. She dressed according to his fashion sense, choosing to wear a similar scarf to that worn by the Fourth Doctor. (TV: 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 said by the Eleventh Doctor. 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 also 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)

Osgood had a sister to whom she negatively compared herself. (TV: The Day of the Doctor) Missy understood that she had little confidence in herself. (TV: Death in Heaven)

Behind the scenes

 * Osgood may or may not be some relation to a Sgt. Osgood who made an appearance as a U.N.I.T soldier / scientist in The Daemons.
 * 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.