Didyme was Aro's younger sister. He turned her when he had been a vampire for around fifteen years, in the hope that a biological relative of his would have a power as useful as his. Unfortunately, she turned out to have the ability to make people happy. Dubbed 'The Walking Prozac', she became a member of the Volturi (which at that point probably consisted of her, Aro and Aro's good friend Mr Blotter Acid).