K is for... Kovarian

Madame Kovarian led a breakaway faction of the Church of the Papal Mainframe. She was at times aided by Colonel Manton of the Clerics, and by fellow members of the Order of Silence. 

Amy Pond began seeing visions of a woman wearing a black eye-patch from the time when the TARDIS visited the United States in 1969. The woman would appear very briefly, as though looking through a small hatch, then vanish again. It transpired that she had been abducted by the woman - Madame Kovarian. She was pregnant and being held on the heavily fortified asteroid known as Demons Run. The Doctor and Rory were initially unaware of this as she had been replaced with a duplicate composed of the substance known as Flesh, which retained her personality and memories. 

Kovarian's first plan had been to blow up the TARDIS, but this plot had failed when the Doctor used it instead to reboot the universe.

Her next plan was to keep Amy's baby, which had Time Lord abilities due to being conceived in the TARDIS whilst travelling through the Vortex. It would be reared to become an assassin, mentally programmed to kill the Doctor. This was because it was feared that he would be responsible for reigniting the Time War, bringing back the Time Lords during his stay on the planet Trenzalore.

The baby grew up to be River Song.

When River failed to kill the Doctor, shattering a fixed point in time, all of Earth's history was damaged. In one of these timelines Kovarian attempted to kill the Doctor using the Silents but her efforts to sabotage the eye-patches of her enemies resulted in the destruction of her own, which electrocuted her. As this was an ultimately aborted timeline, it is not known what her true fate was.

Played by: Frances Barber. Appearances: Day of the Moon, The Curse of the Black Spot, The Rebel Flesh / The Almost People, A Good Man Goes To War, Closing Time, The Wedding of River Song (all 2011).

• Barber is one of Britain's hardest working actors, with some 160 credits on IMDb - and that's just TV and cinema. I recall seeing her in the theatre in the Pet Shop Boys' musical Closer to Heaven in 2001.
• She and Alex Kingston previously appeared together in the credits of the 2003 TV movie The Warrior Queen, in which Kingston played Boudicca, and Barber played Nero's mother Agrippina.