The First Assistant was a person of the City who assisted Senta in draining life energy from the Savages. He prepared Nanina for transference and was left to oversee the process when Senta had to leave the laboratory to deal with an intrusion by Dodo Chaplet. He nearly lost control and called Senta for help and more energy than usual. Afterwards, he was responsible for guiding Nanina outside the city.
He was on duty still later on when Edal ordered Senta to extract life energy from the First Doctor and assisted with the process. Afterwards, he took the Doctor to his guest room. (TV: The Savages)