Osgood
Petronella Osgood was first introduced in the 50th anniversary special as a UNIT scientist and fangirl of the Doctor, known for wearing his most famous outfit choices. During the events of “The Day of the Doctor,” Osgood was one of the UNIT officers copied by the Zygons. When the Doctors made them all forget who was human and who was Zygon, Petronella and her Zygon doppelganger refused to say which was the original, operating as sisters from then on.
The Osgoods were then put in charge of keeping the peace between humans and Zygons and charged with protecting the Osgood Boxes. However, one of the two was killed by Missy, which broke the other’s heart. When another human-Zygon conflict was averted, the Zygon leader took Osgood’s form to continue their dynamic.
For those who only watch the show, Osgood has only had four official appearances. However, the character(s) have appeared in around 50 audio dramas, becoming a vital part of the larger universe. Osgood typically appears in the show when it needs a representative of species co-existence, as the original Osgoods learned from their brief memory loss that they were more alike than they were different.