Gisella

Gisella was in charge of the underwater research base on Flydon Maxima. She had the appearance of a twelve-year-old child with black eyes and dark bobbed hair. She was a mystery, one of a kind. The fact that she never aged was attributed to a unique medical condition termed Adulecentia Perpetus. When her father, Varlos, former Brother of the Darksmith Collective left the Darksmith world on Karagula, he took her with him, hiding her away for her own safety on Flydon Maxima. Following the Doctor's arrival, she accepted his invitation to help him find Varlos and left the research base to join the Doctor on his travels. (TDL: The Depths of Despair)

it is revealed in The Pictures of Emptiness, that she was an android all along. (TDL: The Pictures of Emptiness)