Ada Mullins

Ada Mullins was a Cockney master thief who was responsible for jewel thefts at country houses, for which she became known as "the Unicorn". She assumed the identity of Robina Redmond, "the absolute hit of the social scene," for her stay at Clemency Eddison's house in December 1926.

Biography
During her stay at Clemency Eddison's house in late 1926, 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 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. 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. (TV: The Unicorn and the Wasp)

Behind the scenes
Among the actresses who auditioned for the role was Georgia Moffett, who was cast in the same season's The Doctor's Daughter instead.