The Doctor and Osgood in “The Zygon Invasion”

Which one are you? Human or Zygon?

I don't answer that question.

Why not?

Because there is no question to answer. I don't accept it. My sister and I were the living embodiment of the peace we made. I will give all the lives that I have to protect it. You want to know who I am, Doctor? I am the peace. I am human and Zygon.

Like a hybrid.

A hybrid? If you like.

Well, I'm proud to know you, Osgood. And I promise that I won't tell anyone… that you're a human. Zygons need to keep the human original alive to refresh the body print. If you were a Zygon, you'd have changed back within days of your sister's death.

Those were the old rules. Before Zygons could pluck loved ones from your memory and wear their faces. Zygons only need to keep the original alive if they need more information from them. If the interrogation is over… then the original can die.