Kritakh

Kritakh was a Zygon. He and Torlakh were sent to Earth to make it more habitable to Zygons.

They were to do this by releasing nuclear waste into the atmosphere over a hundred year period. Kritakh took the form of the human Michael Kirkwood as part of his cover. However, over time Kritakh forgot his true identity and believed he was Kirkwood. (HOMEVID: Zygon: When Being You Just Isn't Enough)