The Rani is a renegade Time Lord, an evil scientific genius whose villainy comes not from the usual variety of lust for power and suchlike, but from a mindset that treats everything (including morality) as secondary to her research; she has been known to enslave entire planets such as Miasimia Goria in order to have a ready supply of experimental subjects and a place to carry out her experiments uninterrupted. Her major interest is in tinkering with other species' biochemistry — she was exiled from Gallifrey after some of her lab mice, as a result of an experiment, grew to enormous size and ate the President's pet cat.

She can regenerate, and if her mice could grow enormous, it's likely that she is now big!