John Simm as The Master
The Master has taken many forms over the years.
The Master is one of Doctor Who’s most iconic enemies. Ever since their arrival in season 8 of classic Doctor Who, the Master has been just as important a villain as the Daleks or the Cybermen.
Next to those alien races, the Master really has become the Doctor’s archnemesis. They’ve cropped up again and again over as a constant threat both to the Doctor and to the universe as a whole.
Before the Doctor stole their TARDIS and escaped, the Master was one of their friends on Gallifrey. However, the Master’s continued disregard for “lesser” alien races and constant desire for power drove a wedge between them, forcing them further and further apart.