Captain Jack Harkness is a former time agent who becomes a con man after the agency erases two years of his life.
He meets up with The Doctor in the "Doctor Who" episode "The Empty Child" and continues to travel with The Doctor through "The Parting of the Ways." During the Dalek battle of "The Parting of the Ways" Jack is killed, then brought back to life with the power of the time vortex.
While looking for The Doctor, he becomes trapped in the late 19th century and gets involved with Torchwood, working for them until he takes over.
He later shows up in the 21st century as the leader of Torchwood, a secret government team that fights aliens and monitors a rift in time and space. With Torchwood he is able to use his knowledge and experience to help people, something he wouldn't have done in the old days.
He reveals to new team member Gwen Cooper that he can't die. Apparently, Rose couldn't control the time vortex and brought him back forever. Coming back to life isn't easy, in fact it is sometimes very painful.
Jack grew up in the 51st century on the Boeshane Peninsula with his parents and younger brother Gray. Sadly, Gray was lost during an alien attack and Jack has blamed himself for years. It is the cause of most of his self destructive behavior.
When Gray returns for revenge, Jack accepts his punishment of being buried alive, dying over and over as penance for losing Gray when they were young. This is the vulnerable side that Jack doesn't show very many people.
Jack continues to travel with The Doctor, accompanying him to save the world, fight The Master and a Dalek invasion. As great as he is as the leader of Torchwood, Jack is always at his best when he's with The Doctor.
Captain Jack Harkness is a swashbuckling hero who's not afraid to make tough decisions. He's an omni-sexual flirt who loves life, therefore he'll stop at nothing to protect it. His sacrifices have been many, but he never complains.
He is the reason why The Doctor feels he can leave the Earth. He knows it's in good hands.