Carmen (Planet of the Dead)

Carmen was a middle aged woman, married to Lou, who was onboard the London Bus Number 200 with the Doctor and Lady Christina de Souza during Easter time. She was seen to be a quite caring person to the other passangers and towards the Doctor when she worried for him to be careful after giving him a deadly prophecy. She also seemed to worry quite a bit during dangerous or unexpected situations.

Carmen had low-level psychic abilities. She was able to predict future events (every week, twice a week, she and Lou would win ten pounds on the lottery), sense what was going on without her seeing it, and hear the voices of the dead on a planet. After the bus went through the wormhole to the planet San Helios, she warned the Doctor and the passangers that something was coming, riding on the wind, something that devoured. After the Doctor saves her and they return to Earth through the wormhole, Carmen tells him a prophecy: "...your song is ending, sir... it is returning through the dark... and ... oh, but then... he will knock four times." (DW: Planet of the Dead)

Her prophecy would continue to haunt him through his travels, as seen when he mentions it to Adelaide Brooke and Wilfred Mott, and uses electricity on the Flood before it can knock a fourth time, saying, "three knocks is all you're getting!" (DW: The Waters of Mars, DW: The End of Time) Despite the Doctor's attempts to avoid Carmen's prophecy coming true, in the aftermath of defeating the Time Lords, he the hears four knocks, and realises it was Wilfred all along. Wilfred's knocking does ultimately lead to the Tenth Doctor's death, in which he became the Eleventh Doctor. (DW: The End of Time)