Jack Harkness is a fictional character from the British television series Doctor Who and its spin-off, Torchwood. He is portrayed as a time-traveling former Time Agent with a charming personality and a mysterious past. Jack is known for his immortality, which allows him to come back to life after dying, making him a unique and complex character. In Torchwood, Jack serves as the leader of a team that investigates extraterrestrial incidents and protects Earth from alien threats. His character often grapples with themes of loss, love, and sacrifice, making him a compelling figure in the Doctor Who universe.