Captain 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 survive injuries that would be fatal to others. In Torchwood, Jack serves as the leader of a team investigating extraterrestrial threats in Cardiff. He is characterized by his bravery, resourcefulness, and a strong sense of duty. Jack's complex relationships with other characters, including Ianto Jones and Gwen Cooper, add depth to his story throughout the series.