Berenyi was a Time Lord. She graduated from the Academy with a degree in bio-chronology, and "disappeared" from Gallifrey to become a freelance Designer and began rewriting the timeline around events. She also appreciated the irony of the profession since designing time had been banned by the Time Lords centuries before. She did it more out of spite than anything. Berenyi was one of a number of other renegades who had grown tired of the restrictive nature of Gallifrey.
As a mercenary hired by wealthy clients to alter time, her work on "an assassination in a theatre in 1865" had attracted the attention of a German from the 22nd century, who hired her to go to 1963 and design the assassination of John F. Kennedy. She considered 1963 mundane. She programmed the life of a human male to give him the right background, spending a lot of her time on him and his programming — who had been the chairman of the Fair Play For Cuba committee and who had a Russian wife — to shoot the president. Berenyi had designed his past meticulously to make him the perfect assassin and she was impressed by her work with him, showing that she took great pride in her work.
As a part of her "design", she had established that Jack Ruby would kill the president's assassin.
A red-headed smoker at the time of the Kennedy assassination, Berenyi was also given to wearing exclamation points, perhaps indicating she knew of the Doctor's affinity for wearing question marks. Like the Doctor, she possessed a fairly detailed knowledge of Earth culture and history, and she seemed to have an interest in learning more about humanity. She liked coffee. She was known to like The Jesus and Mary Chain, Sonic Youth, Cherry 7UP and Arthur Miller's biography.
She also had her own TARDIS which she used to escape Gallifrey and to also alter the timeline. It was disguised as a filing cabinet in the book depository from which Kennedy's assassin operated. When her work was finished, she entered it and left. (PROSE: Untitled)