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. (PROSE: Halflife)