Helen Kristiansen was the Divine Empress of the Earth Empire in the 30th century.
She was first President of Earth, before becoming Empress. Her coronation happened sometime around the mid 28th century. (PROSE: So Vile a Sin) She ruled the Empire from her palace orbiting Saturn. She was extremely old, with tumours covering her body. She was fully integrated into the computer systems that ran the empire. (PROSE: Original Sin)
Finally tired after ruling for so many years, she asked the Seventh Doctor to end her life, since his unaligned status meant he was the only one who could do so without political ramifications. He did so on 3 June 2982. (PROSE: So Vile a Sin)