Zygon Osgood

At least two Zygons adopted the identity of Petronella Osgood at various times.

Biography
Part of a group stuck on Earth, the Zygons landed in 1562, but the group retreated into a stasis cube and stayed there until the 21st century. After leaving the stasis cube, now housed at the Under Gallery, one Zygon took Osgood's form after ambushing the real Osgood and took her inhaler. The real Osgood escaped, however, and reclaimed her inhaler while doing so.

Later, three incarnations of the Doctor erased the memories of both the humans and their Zygon counterparts when they took control of the UNIT Black Archive, forcing the two species to negotiate a ceasefire. When the Zygon Osgood suffered an asthma attack, the real Osgood quietly passed over her inhaler, allowing these two to secretly know which was human and which was Zygon. (TV: The Day of the Doctor)

The two Osgoods then created Operation Double, where a group of 20 million Zygons would be allowed to live on Earth and take on the form of the nearest person they could find. The treaty could only be maintained if both humans and Zygons agreed to live among each other in peace, and that no harm would come to anyone. They were given a device called the Osgood Box by the Doctor, to be used only in a dire emergency. (TV: The Zygon Invasion)

In 2014, one of the Osgoods was sent to Bonnybridge to investigate another U.F.O. sighting in the area. Because of this, she missed the first known interaction between the Twelfth Doctor and UNIT as well as the Fractures' attack on London. (COMIC: The Fractures)

Death
After began to deploy her Cyberman army across the world, (TV: Death in Heaven) the Zygon Osgood (AUDIO: Narcissus; WC: The Zygon Isolation) confronted and secured her. She was stationed near Missy during her captivity aboard Boat One, who told Osgood she was planning on killing her. Missy proceeded to count down to Osgood's death, taunting her with each number. She then unlocked 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)

Legacy
After an encounter with the Twelfth Doctor, another Zygon, Bonnie, decided to become an Osgood, taking the place of the one who had died. Neither Osgood would reveal if they were human or Zygon. (TV: The Zygon Inversion)

Behind the scenes

 * The Zygon Osgood was the first recurring character to be actively killed off in the 2005 Doctor Who, although other characters, such as the Face of Boe, Brigadier Lethbridge-Stewart, Amy Pond, and Rory Williams had been depicted as dying of old age, Jack Harkness and The Doctor himself had suffered multiple deaths and resurrection, and River Song is a special case due to her asequential timeline compared to the Doctor.
 * Additionally, for a while it was left purposefully ambiguous which Osgood was killed off, and has still never been stated on television, but expanded media has confirmed that it was indeed this version of her that was killed.
 * However, she is predated by Harriet Jones if non-televised media is discounted, as no television story thus far has confirmed her survival of her seeming extermination by the Daleks as shown in Harriet Jones, PM

Osgood Zygon