The Doctor is a character played by Kris and the title character of the BBC's Doctor Who, though he is never called that. He is somewhere around 900 years old and is a member of a humanoid alien race called Time Lords. He travels through space and time in his TARDIS.
One of the Time Lords' most distinctive features is their ability to "regenerate": to die in one body and be "reborn" in the next. The transformation takes place within a matter of minutes; one body disappears as the other appears. The current Doctor is number 10 (out of a possible 12). With the new body come variations in personality; so each Doctor is somewhat different from the last, though with a shared memory and certain core traits.
This Doctor, played by David Tennant, is tall and thin with brown hair and eyes and perpetually dressed in suits and Converse sneakers, sometimes with glasses.