Christina de Souza

Christina de Souza is a jewel thief with aristocratic manners and a taste for spectacle, first seen scaling the outside of Buckingham Palace to steal the crown jewels. She's cool under pressure, quick-witted, and utterly unbothered by the Doctor's usual awe-inspiring effect on people. She wants freedom, wealth, and the thrill of getting away with it.

Michelle Ryan plays her as someone who's already the most interesting person in any room, which makes her a rare match for the Doctor rather than a follower. The writing gives her competence to burn — she can out-plan, out-run, and out-talk most threats — and Ryan delivers the posh bravado with just enough mischief to make it charming rather than grating.