The Rani

The Rani is another renegade Time Lord (or Time Lady); the Rani attended school with both the Doctor and the Master, and is the same age as the Doctor (953 at the time of his regeneration into the Seventh Doctor). Unlike the Master, the Rani uses power as a means to an end-that being whatever research the scientifically-minded villain is engaged in at the time. Other beings mean nothing to her except as subjects of her experiments. This tendency goes back at least to her University days, where she reportedly mutated mice into monsters, which ate the President's cat. In fact, the Sixth Doctor's regeneration into the Seventh Doctor was directly responsible to stop her from recreating the Universe in her own image.

The Rani was in The Mark of The Rani and Time and The Rani.