The anonymous time traveler we call "The Doctor" was born on the planet Gallifrey in the constellation of Kasterborous, to a Gallifreyan father and a human mother. Physically he is much like other Gallifreyans, with two hearts which pump recognizably non-human blood, and a respiratory bypass system which enables him to recycle oxygen for a brief time. The only apparent physical characteristic which he inherited from his mother is his eyes. Although their color may change when he regenerates, they retain a human retinal pattern. This may explain his occasional need for spectacles or magnifying glasses, which never seem to be used by full-blooded Gallifreyans.

The planet Gallifrey is much like Earth, but some of the trees have silver leaves, and its night sky is a burnt orange color. The Doctor has memories of lying on a hillside with his father while watching a meteor shower in this night sky. Aside from his granddaughter, Susan, and an uncle he once mentions, we hear of no other members of his family. When speaking to his companion Victoria Waterfield about her father's death, he says that his family sleeps in his mind, and that it takes an effort for him to remember them. He tells Kathleen Dudman, the future grandmother of his companion Ace, that he is not sure if his family is alive. The Doctor has been only slightly more forthcoming about his childhood. We do know that he lived in a house in the mountains of South Gallifrey, near a hermit who told him ghost stories, including those of the Fendahl and the Great Vampires. Omega and the Time Lord renegade Salyavin were among his early heroes. He had a boyhood dream of driving a steam engine. His childhood games included conkers, trains, and hopscotch, and his ability with a yo-yo suggests lots of practice. At some time in his childhood, he was accepted at the Prydonian Academy, and began the studies which would make him a Time Lord.

Throughout the eight incarnations whose adventures have been recorded in detail, The Doctor has exhibited a variety of personality quirks, interests, and abilities, some of which change, or seem to be forgotten, from one regeneration to the next. There are, however, some elements which are common to all the Doctors.

The Doctor has a brilliant, if undisciplined, mind. He is passionately interested in many subjects, especially science, history, art and literature. He is insatiably curious, to the extent that he has often endangered himself and his companions in his quest for knowledge. He is a born meddler who seems incapable of walking away from other people's problems. Through all of his lives he has been somewhat arrogant, and he does not suffer fools gladly, though he is usually patient with those who are willing to learn from him. He generally carries an assortment of odd items, some of which seem to be completely useless, until one of them turns out to be exactly what's needed to get him out of a sticky situation.

Though the TARDIS contains wardrobe rooms which are filled with costumes from many times and places, his peculiar sense of style means that he rarely makes use of them. Once he decides on clothing which suits his current personality, he seldom deviates from it unless forced to do so. Despite this, he has the uncanny ability to blend in almost everywhere he goes, so much so that people often don't notice his strange attire. He has an amazing ability to ingratiate himself with people who were suspicious of him only minutes before. Sometimes he achieves this by solving a problem for them, but on many occasions it seems to be simply the result of his charismatic personality.

Though he is often impatient with his companions, he has great affection for most of them, and has willingly risked his life for all of them on many occasions. Probably the most important constants about The Doctor are his desire for justice, his willingness to put himself on the line to defeat evil wherever he finds it, and, of course, his amazing ability to find that evil wherever he goes.