The Master

The Master is a Time Lord and arch-nemesis of the Doctor. He tries many nefarious deeds and alliances to try and kill the Doctor, including allying himself with the Cybermen and The Daleks, and also attempting to transform all humans into duplicates of him and trying to crash assorted large objects into the planet.

The Master is insane, and is constantly tormented by a drum noise in his head. He blames his insanity on the constant rhythm, and tries everything he can to stop it. he thinks killing the Doctor is somehow the key to finally ending the constant sound of drums.

The Face of Boe predicted the Master's return from the Time War, an eternal war between the Daleks and the Time Lords that was locked out of time, and only Dalek Caan was able to enter it, and it cost him his mind. The Master unwittingly transformed into a human called Yana, who was nice anf friendly until he opened the watch continaing the Master's soul, then the Master was reborn. The Face used his last breath to predict this, saying to the Doctor 'You Are Not Alone' (YANA), hinting at the old man's true identity.

He knew where his loyalties truly lay, however, when the insane leader of the Time Lords, Rassilon, was brought back to Life. He is revealed to be the one responsible for the Drumming, as it is all part of his plan to bring Gallifrey, the planet of the Time Lords, back to full might by using humans as slaves.

The Master is not too happy about this, however, and beats Rassilon up with his newfound powers, saving the Doctor but dying in the process.

Youtube Clips
http://www.youtube.com/watch?v=OF_yjfCVpRo

http://www.youtube.com/watch?v=m0RvDZFN71A

http://www.youtube.com/watch?v=8fWvexyy37w

http://www.youtube.com/watch?v=Cfyye1rTfdg