Madame Xing was a being encountered by the Eighth Doctor on Espero.
Roberto told The Doctor that Madame Xing was an offworlder to Espero. He said that no one knew her true name, because it was unpronounceable.
Xing claimed to be a viropractor, specialising in the use of viroids to alter and enhance cognitive development and memory. She helped The Doctor restore the memories of his arrival on Espero. She then offered to restore the remainder of his memories, to which he refused. (Halflife)
After discovering why he had lost all of his memories, The Doctor sent K9 to Espero. (The Gallifrey Chronicles)
Xing wore all black clothes, with a hood hiding her head, and black gloves covered her fingers. Her voice sounded to The Doctor like it had been augmented by machinery. (Halflife)
BEHIND THE SCENES
Madame Xing knows who The Doctor is; she knows about the Time Lords despite their erasure from history; and she’s implied to be dimensionally transcendental. This could describe either Compassion or Countess Gallowglass from the author’s previous novel, Relative Dementias.