Constable Pavo was a member of Chapter 9, the Time Lord police force.
Pavo was pursuing the Monk and caught up with him at a research station. However, the Monk shot him with Pavo's own baton. The Monk then stole Pavo's perception-altering ring in order to impersonate him.
The Monk thought that he had killed Pavo, but with the help of the Second Doctor and Jamie McCrimmon, he was able to regenerate. Pavo's new incarnation was a woman, who assumed the alias of "Commander Melanie Flail".
Pavo intended to also arrest the Doctor, but eventually, after the Doctor helped her defeat the Monk and a Seeth invasion, she decided to let him go this time, after hypnotically wiping the Doctor, Jamie and Victoria's memories of the event, 'officially' claiming that this would help her escape the taint of requiring the aid of a renegade to resolve this crisis. (AUDIO: The Black Hole)
Appearance
One incarnation of Pavo was described as a bald man. (AUDIO: The Black Hole)
Behind the scenes
Although never stated in the story, Simon Guerrier intended for the two Pavos to be earlier incarnations of the Master.[1]