Collins (The End of Time)

Miss Collins worked for Joshua Naismith in the 21st century. When she was a child, an accident left behind some burns down her left side. After the Immortality Gate was tested on her, these burns were healed. Like the rest of humanity, save two, she was turned into a member of the Master Race and was restored to normal by Time Lord President Rassilon. When Gallifrey appeared in the sky above Earth, Miss Collins fled the mansion in terror. Her employer was later arrested for "crimes undisclosed". (TV: The End of Time)

Behind the scenes
The script describes Ms Collins as 21 years old.