The Leader

In a parallel universe, the Leader was the President of the Republic of Great Britain and mistakenly believed to be the third incarnation of the Time Lord known as the Doctor. Based solely on the Doctor recognising the face on the posters as one of those offered to him by the Time Lords. (PROSE: Timewyrm: Revelation)

Biography
Based on the Doctor recognising the face on the 'Big Brother' posters as one that he was offered at his trial, the Doctor mistakenly believed the Leader was an alternative incarnation of his. The Seventh Doctor offered no proof to back this supposition up. (PROSE: Timewyrm: Revelation, TV: The War Games)

The Leader was revealed to be an alternative version of the Brigadier's father, Gordon Lethbridge-Stewart. (PROSE: Night of the Intelligence) Another example of Time Lords using faces of existing humans. (TV: The Woman Who Lived)

The Leader may have been at least partly responsible for his Earth's much more advanced technology. He died in the volcanic catastrophe that struck Great Britain in the aftermath of I-Day. (PROSE: The Face of the Enemy)

Behind the scenes
Photos of the BBC special effects designer Jack Kine were used to depict the Leader on the UNITY IS STRENGTH poster in the Technical Stores (the Doctor's hut on the regular Earth timeline) and the framed photograph on the wall of Brigade Leader Lethbridge-Stewart's office in Inferno. However, the Leader never appears on-screen in person and there is no on-screen evidence to suggest that the Doctor and the Leader had any connection; this connection was made only in the novels by the Doctor who recognised the face. The Lethbridge-Stewart series has confirmed, however, that it was the Inferno-Earth version of Gordon Lethbridge-Stewart.