People and characters that the Doctor has met during his travels

Brain
The Brain, used by the Rani to calculate the right launch time and trajectory for her missile
Channing
Channing, a figurehead for the first Auton Invasion
Lytton
Lytton, a mercenary for hire who worked for the Daleks and against the Cybermen
Sil
Sil, one of the evil Mentors
The Master
The Master, another renegade Time Lord
The Rani
The Rani, a renegade Time Lady from Gallifrey
The Valeyard
The Valeyard, the dark side of the Doctor, somewhere between the twelth and thirteenth regenerations