The Doctor is the main hero of the popular TV show Doctor Who as well as the many comic books and spin-offs spawned from its success - he is an alien who protects the world from such menaces as the Daleks and The Master and is famous for his TARDIS, sonic-screwdriver and ability to regenerate into a new form with a slightly different personality if he is on the verge of death.
History
The Doctor is an alien. He is the last surviving member of a race known as the Time Lords from from the planet Gallifrey. He travels through time and space in a spacecraft called the TARDIS, frequently with human companions, averting any crisis he comes across using science, technology, and wit. As a Time Lord, he has the ability to regenerate, or change his appearance and personality when on the verge of death. Although the series is titled Doctor Who, this is not the character's name: he is only ever referred to as The Doctor and his real name remains unknown.
Abilities
Regeneration: When a Time Lord is dying, they are able to use a process called regeneration. Regeneration is a process in which all the cells in the body are replaced with new ones. The result is a completely new appearance. Even though the Doctor has changed ten times, he still retains the same memories of his previous selves and remains a force for good, even though his priorities and personality sometimes change.
A Time Lord can only regenerate 12 times, for a total of 13 incarnations. With the current series now on the 11th Doctor, it is assumed that the Doctor can only regenernate two more times. However, the Time Lords, who controlled regeneration were established as being able to grant additional lives beyond the 13 if they so wished; also since they no longer exist, their social order has been destroyed, so potentially the Doctor can regenerate indefinitely. An episode of the spin-off series "The Sarah Jane Adventures" has the Doctor state he can regenerate 507 times, but this is not to be considered an official number.
First Doctor
The First Doctor
The First Doctor was played by William Hartnell.
The First Doctor was the youngest incarnation of the Doctor, but had the most aged physical appearance, having grown old through a natural lifespan. This Doctor was an unreadable, guarded figure who was, at first, slow to trust newcomers who learnt of him. This Doctor was often irascible. He made his anger obvious. He was protective of the young women he took on as companions; they reminded him of his granddaughter, Susan. This Doctor was a brilliant, often short-tempered scientist and keen strategist. He used his signet ring to help get himself through ordeals due to his physical age impeding him. He stole a TARDIS and took his granddaughter with him, joyriding through all space and time, without a clue as to how to drive the thing.
He played deadly games with the Celestial Toymaker, he fooled Roman emperors, mongol warlords and French revolutionaries, and he foiled each and every machination of the Daleks. After preventing the Cybermen from absorbing the Earth's energy and with his body “wearing a bit thin”, the First Doctor changed into his second incarnation.
Second Doctor
The Second Doctor in contrast to his preceding incarnation, was warm and wise. He was as surprised and frightened of alien menaces as those who faced them with him. He had a knack for manipulation and deception. His predecessor would refer to him as a "clown" due to his bumbling nature. He loved tootling on his recorder and carried around a 500-year diary, trying to record his travels, but ended up discarding it. He wore a big fur coat that dwarfed him, tying it closed with twine. A "cosmic hobo," he was forever getting himself in and out of trouble.
Third Doctor
Fourth Doctor
Fifth Doctor
Sixth Doctor
Seventh Doctor
Eighth Doctor
Ninth Doctor
Tenth Doctor
Eleventh Doctor
The 11th and current Doctor.
Darker Doctor
This incarnation was between the Eigthth and Ninth Doctors but did not hold the moniker of "the Doctor" due to his actions in life. This unknown incarnation responded by defending those actions, stating he had "no choice" and that he did what he did "In the name of peace and sanity".