The Second Assistant was a person of the City who assisted Senta with life force extractions. He assisted with the extraction from Nanina and spotted Dodo Chaplet hiding in the laboratory. He captured her with help from the Third Assistant and, even though she was not on their list, assumed she was one of the Savages and tried to prepare her for extraction.
When she fought back, he went to get Senta. Dodo explained she was one of the time travellers they had heard about and she was taken away by Edal. The Second Assistant later assisted with the extraction performed on the First Doctor. (TV: The Savages)