Margaret Hopwood

Margaret was a companion of the Doctor.

She was captured by the Nine and placed in a prison along with various other companions. She could be heard bemoaning the fact that she was not young any more, and begged to be let out. (AUDIO: Companion Piece)

Behind the scenes
Margaret has not appeared in any other stories and Companion Piece never states which Doctor she travelled with. Her corridor she was kept in implied she may have been a companion to the Fourth Doctor. This has been further implied by the fact that Nerys Hughes will be playing a character called in a future Fourth Doctor Big Finish story, and then later playing that same character as a companion of the Fourth Doctor.