The Doctor is a space and time traveling alien, the protagonist in the fictional Doctor Who universe, who was created in 1963. Currently the Doctor is in its 13th incarnation, which is the form of a woman, a first.
The Doctor is a renegade Time Lord, who has vowed to do no harm and to help others. Whom has traveled across the cosmos, across time, and to other dimensions and realities. His two favorite worlds are his homeworld Gallifrey (the home of the Time Lords) and Earth, both which he defends ferociously. The Doctor has saved numerous civilizations, species, worlds, and galaxies, in addition to the many individuals whose lives he has saved. It's unknown how old the Doctor truly is due to all the time traveling and temporal anomalies / time locks the Doctor has experienced, but the Doctor claims to be over 800 years old.
The Doctor has fought many tyrants, empires, organizations, criminals, races, and entities both on Earth and in space. Of which it's greatest enemies are the Daleks, a race of intergalactic genocidal xenophobic cybernetic mutants whom pose the greatest threat to the universe. The Doctor has also fought several entities of the Mythos including Hastur and Yog-Sothoth, prevailing every time so far.