The Master is a renegade Time Lord and the Doctor's arch-nemesis.
Although they were originally boyhood friends, one of the Master's primary goals was to destroy the Doctor and Earth while acting on a number of schemes, both petty and gross. It was eventually discovered that the Master's diabolical madness was partially the result of a genuine malady in the form of a never-ending drumming sound that had been retroactively implanted inside his head by the Time Lords on the last day of the Last Great Time War to further their own goals.
- Read more about this topic on the Tardis Data Core.
There are currently 5 incarnations of the Master in Doctor Who: Legacy.
The ability icons of the Master are deliberately different from those of other enemies, in order to confuse players.