Captain Jack Harkness

Before meeting the Doctor, Captain Jack Harkness was a Time Agent from the 51st century. He grew up on the Boeshane Peninsula and was the first ever to sign up for the Time Agency. While working as a Time Agent he had two years of his life stolen, so as revenge, he became a freelancing conman.

Taking on the fake name Captain Jack Harkness, he decided to con other Time Agents in the World War II time period, where he spotted Rose hanging from a blimp. He rescued her and together they found the Doctor (who was in a hospital examining gas-masked zombies). Jack then admitted that he was a conman, and it was revealed that his con was the cause of the gas-masked zombies. The Doctor saved the day, naturally, by reprogramming the nanogenes that had caused the gas-masked zombies in the first place to change everyone back. Then a german bomb was dropped upon on top of them.

Jack stopped the bomb from crashing on top of the Doctor and Rose by bringing it into his space ship. The Doctor then rescued Jack from the bomb (which was still going to explode).

Jack accompanied the Ninth Doctor until the end of series one, when he was exterminated by Daleks. Rose, returning to the Gamestation to safe the Doctor as the Bad Wolf, brought Jack back to life, accidentally bringing him back to life forever, making him immortal; a fixed point in time. The Doctor, unable to look at Jack without feeling extremely uncomfortable, then abandoned him in the year five billion.

Using his vortex manipulator, Jack landed in Earth, eighteenth century, where he discovered he could never die. He waited for decades until the Doctor returned (joining Torchwood in the mean time) and finally managed to catch the Doctor. The Tenth Doctor tried to get away from him, but Jack jumped and clung on to the outside of the TARDIS. The TARDIS travelled all the way to the end of the universe trying to get rid of Jack.

The Doctor, Martha, and Jack met Professor Yana, who later turned out to be the evil and insane Time Lord the Master.

The Tenth Doctor with the Face of Boe

The Master imprisoned Jack on a flying ship called the Valiant during the Year that Never Was (because the Doctor reset everything). After this unpleasant experience, Jack turned down the Doctor's offer to begin traveling with him again and returned to his life on Earth.

Before leaving, Jack admitted that despite being immortal, he was still aging very slowly, and asked what would happen if he lived of a million years. The Doctor could not give an answer to this. Jack then revealed that he had once gone by the name "The Face of Boe", who was a strange creature the Doctor had met several times on his travels.