Captain Jack Harkness" was the alias adopted by Time Agent and con man Javic Piotr Thane from the 51st century who became an associate and occasional companion of the Doctor.
After he was killed by a Dalek, he was revived by Rose Tyler, who at the time was transformed into a nearly omnipotent being. Unable to control her powers, she accidentally turned him into an immortal being. Because of this, the Ninth Doctor saw his companion as a fixed point — a temporal paradox that threatened the TARDIS' proper operation. Consequently, the Doctor abandoned him in the year 200,100 aboard a space station orbiting a Dalek-devastated Earth.
During his long wait for the "right Doctor", he experienced what was to him Earth's history first hand. He also had many different relationships, some of which produced offspring. For the majority of those years, he also worked for Torchwood Three. For several decades an informal free-lancer, he eventually became its head in the year 2000. Later that decade, he finally met up with the Doctor, although the Time Lord had by this time regenerated, and was now traveling with Martha Jones.
Powers and Stats
Tier: 9-C, 9-B (With Sonic Blaster), 9-A (With Energy weapons)
Name: Jack Harkness (Captain Jack Harkness)
Origin: Doctor Who
Attack Potency: Street level, Wall level (With Sonic Blaster), Small Building level (With Energy weapons)
Speed: Normal Human level
Lifting Strength: Regular Human level
Striking Strength: Human Class
Durability: Human level, although immortality makes him almost impossible to kill
Range: Several meters with his equipment
Standard Equipment: Gun, other regular weapons. Vortex Manipulator, Sonic Blaster, Energy weapons.
Weaknesses: Becomes mortal on "Miracle Day".