The Rani

Ushas, better known as the Rani, was a renegade Time Lord. She knew the Doctor and the Master when all three were young, and became an enemy of the former and an unwilling ally of the latter.

The Rani was a brilliant scientific genius whose villainy came not from the usual variety of lust for power and suchlike, but from a mindset that treated everything (including morality) as secondary to her research. She was highly intelligent but extremely arrogant, narcissistic, ruthless, powerful and intensely cruel. She was known to enslave entire worlds in order to have a ready supply of experimental subjects and a place to carry out her experiments uninterrupted. Her major interest was in altering the biochemistry of other species.