Sophia was a human/Hetrodon hybrid and a time sensitive who lived in 540. She was employed by the Monk to help him in his plans to get revenge on the Eighth Doctor after he caused Tamsin Drew's death. She told Vicki Pallister that Steven Taylor was taken to Constantinople. The Monk took her from Nicopolis in Greece. She released the Ostardi which attacked Justinian and Theodora. (AUDIO: The Secret History) The Monk used her powers to divert the Seventh Doctor to take the place of the Third Doctor battling the Europans on Delphin Isle in the 1970s (AUDIO: The Defectors) and replaced the Second Doctor with the Sixth Doctor on a planet in the Kuiper belt to battle the Cybermen in the 26th century (AUDIO: Last of the Cybermen) and finally to move the Fifth Doctor to Constantinople in 540, replacing the First Doctor. this action allowed the Monk to replace the Doctor in his timeline.
She eventually realised the error of her ways and restored the timeline. She then decided to help with the plague. (AUDIO: The Secret History)