When the young Kelly encountered a man who had been splintered in time, her sensitivity made him feel wrong to her, so she stabbed him to death. This also killed the other seven fragments of the man, which all appeared to be children. Due to their connection, the knife wounds on all eight were identical. She was convicted of murder and placed in an asylum for the perceived heinousness of her crimes.
Sabbath rescued Kelly, which made her extremely loyal to him. When Octave tried to kill the Eighth Doctor, Sabbath collapsed due to the connection between them from having one of the Doctor's hearts in his chest. Kelly unwittingly performed CPR on Sabbath by beating on his chest in desperation, which saved the Doctor as well.
Ironically, the Doctor later tricked her by letting her overhear him say he believed Sabbath was a threat that had to be dealt with. When she attacked him, he made sure that she stabbed him in the heart. This was so he could enter the realm of death and communicate with the spirit of Sebastian Chiltern. Kelly watched over the comatose Sabbath until he and the Doctor recovered.
When the Doctor tried to destroy the temporal interferometry machine, Nathaniel Chiltern attempted to prevent him. Kelly intervened, so Chiltern killed her. Sabbath then killed Chiltern. (PROSE: Camera Obscura)
Behind the scenes
Elizabeth Kelly appears in Daniel O'Mahony's A Rag and a Bone, published in the fanzine Myth Makers Presents: Essentials in 2003. The story is a metafictional commentary on Doctor Who's state at the time, Kelly representing Sabbath's companion and lover, the exact idealogical and symbolic opposite of Anji Kapoor, the Eighth Doctor's companion and friend.