Captain Jack Harkness played by John Barrowman
Captain (not a real captain) Jack Harkness, a former Time Agent, was an intergalactic conman; he was on Earth trying to con the Doctor, and was on hand to save Rose as she fell from a barrage balloon. He was willing to sacrifice himself and his stop in order to stop a Word War 2 bomb dropping in London, but was rescued by the Doctor.
After experiencing the Doctor's lifestyle he left his conman persona and became an action hero. He was killed by the Daleks, but brought back to life by Rose. The Doctor, thinking he was still dead, left the space station just before Jack could get to him. He later found his way to Earth and became the leader of Torchwood in Cardiff. When the TARDIS landed in Cardiff he rejoined the Doctor after clinging to the outside of the TARDIS.
Rose gave the gift of life back to Jack; Jack can be killed but always reawakens.