"The Doctor" was the main alias used by a renegade Time Lord from Gallifrey who travelled through time and space with various companions in their obsolete and "borrowed" Type 40 TARDIS. They were the universe's "greatest defender", having saved the cosmos thousands of times throughout a long life, becoming a great legend across the whole universe.
Though largely a believer in non-violent conflict resolution, they were, when absolutely necessary, a great warrior. Indeed, some civilisations in the universe (e.g. the denizens of the Gamma Forests) translated the word doctor as warrior, (TV: A Good Man Goes to War) whilst others saw the Doctor as a compassionate benefactor, worthy of their admiration and compassion. (TV: Last of the Time Lords, The Wedding of River Song)
Although they had saved untold numbers on their travels, the Doctor was thought to have caused the deaths of billions at the conclusion to the Last Great Time War. (TV: Dalek) Though most of the Daleks were killed in the crossfire, Gallifrey — disappearing, rather than being burned — was hidden thanks to the efforts of the first thirteen of their incarnations, the first eleven of which retained no memory of the event. (TV: The Day of the Doctor) For their actions, the Time Lords granted the Doctor a new regeneration cycle, allowing them to live on after using up all available regenerations in their first cycle. (TV: The Time of the Doctor)
From the latter years of their first incarnation onward, the Doctor had a pronounced affinity for Earth and the human race. After departing Gallifrey, they voluntarily chose to spend time on the planet, (TV: An Unearthly Child, AUDIO: Summer, The Haunting of Thomas Brewster) choosing it as the place of their exile during most of their third incarnation, (TV: Spearhead from Space - The Three Doctors) and even owning property in Kent (COMIC: Fellow Travellers, PROSE: Verdigris, Warlock, Warchild, The Dying Days, Mad Dogs and Englishmen) and New York City. (PROSE: The Forgotten Army) They favoured Great Britain as a frequently visited location, taking most of their companions from there. (TV: An Unearthly Child, Spearhead from Space, The Time Monster, Rose, Smith and Jones, Partners in Crime, etc.) Even before the disappearance of Gallifrey, the Doctor spent much more time on Earth than on their homeworld.