Captain Arisa Magambo was a female officer working with UNIT in the early 21st Century. She was part of the operation to remove the insectoid entity on Donna's back. She was in charge of a special division of UNIT in the alternate timeline. She with the help of Rose Tyler, sent Donna back to prevent the events which would result in the Doctor's death and eventual world-wide disaster.
It is not known what happened to her real timeline self.
Behind the Scenes
She is the second female UNIT captain to appear in the fourth series, the first was Captain Marion Price