Ace (Sophie Aldred)
When the Doctor first met Ace she was working as a waitress on the planet Svartos, after being plucked from her home in Perivale by a freak time storm. She had a rebellious streak, and aptitude for explosives, the result of a troubled upbringing.
At the age of thirteen she'd burnt down a malevolent Victorian house, (a place she'd eventually revisit with the Doctor), and her best friend, Manesha, had been the subject of a racially-motivated arson attack.
Ace matured considerably during her travels with the Doctor. Their last adventure together was on the planet of the cheetah people.