Ada Mullins was a Cockney master thief who was responsible for jewel thefts at country houses, also known as The Unicorn
Biography
Ada was born circa 1898. (PROSE: The Time Travellers' Almanac)
She planned once to steal the jewel known as the Firestone, so posed as celebrity Robina Redmond, to stay at the house of the jewel's owner, Lady Clemency Eddison.
During her stay, the murder of Professor Gerald Peach disrupted her plans. She was questioned by the Tenth Doctor and Agatha Christie along with the other guests, where she made the mistake of referring to the loo as the "toilet", cluing the Doctor onto her identity as the Unicorn. Believing that Donna Noble might find the kit containing the tools of her trade, Ada hurriedly tossed them into the garden, where they were later found by Donna and Agatha.
After the murders of Miss Chandrakala and Roger Curbishley, she swiped the Firestone and hid it down her décolletage in the confusion.
However, during the Doctor and Agatha's solution of the case, Ada had her identity revealed and she returned the Firestone to Lady Eddison; when Donna asked if she was also the murderer, Ada confessed she was only a thief, and did not murder anyone that day.
After Reverend Arnold Golightly was revealed as the long-lost hybrid child of Lady Eddison and a Vespiform called Christopher, she escaped in the confusion, heading back to London, but without the Firestone. (TV: The Unicorn and the Wasp)
Behind the scenes
Among the actresses who auditioned for the role was Georgia Moffett, future wife of the Tenth Doctor's actor, David Tennant, and daughter of Peter Davison, the Fifth Doctor. She was cast in the same season's The Doctor's Daughter instead, as the titular character Jenny.