Robina Redmond/The Unicorn

Ada Mullins, aka "the Unicorn" was a Cockney master thief who was responsible for jewel thefts at country houses. 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. However, the death of Professor Peach disrupted her plans. She was questioned by the Tenth Doctor and Agatha Christie along with the other guests. Believing that Donna Noble might find the kit containing the tools of her trade, the Unicorn tossed them into the garden, where they were found by Donna and Agatha.

She made an error in word choice during her interrogation; she referred to the loo as the "toilet". After the murders of Miss Chandrakala and the Hon. Roger Curbishly, she swiped the Firestone. However, during the Doctor and Agatha's solution of the case, the Unicorn had her identity revealed and she returned the Firestone to Lady Eddison. 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.

Biography from the TARDIS Data Core article, licensed under CC-BY-SA