He only appeared in a handful of episodes in the first season of Doctor Who, and from there on, his appearances in the show were rather sporadic. However, Captain Jack Harkness remains one of the most popular companions among Doctor Who fans. He was such a beloved character that he achieved something only one other companion managed to do: get his own show.
After the Doctor left him, Jack joined the Torchwood Institute and eventually went on to lead the Cardiff branch of Torchwood. It helped that he had become immortal and could work for Torchwood for over a hundred years. Considering his long life, there are plenty of interesting details viewers might not know about Captain Jack Harkness.
5 Openly Pansexual
A common misconception about Captain Jack Harkness is that he's bisexual. However, that's not entirely accurate. Jack likes to flirt with and date other beings regardless of their gender. He demonstrates this repeatedly by mentioning his more romantic adventures with aliens of various races, who are unlikely to have the same gender concepts as humans do.
Even on the human side, Jack is free with his affection and picks partners based on their personality and appearance, not their gender. He kisses both Rose and the Doctor, flirts with Gwen Cooper, forms a long-term relationship with Ianto Jones (until Ianto dies). He kisses the real Captain Jack Harkness from whom he borrowed his name, but he also fathers a child in the 20th century. As the first openly pansexual character in Doctor Who, Jack shows that the future is no longer as concerned with gender differences as it once was.
4 His Childhood And Family
By the time the Ninth Doctor and Rose encounter Jack for the first time, he's already a con man who's using an assumed name. However, throughout the Doctor Who and Torchwood series, viewers get more details about Jack's past and his family, as long as they're paying attention. One of the first things the Doctor learns about Jack is that he was born in the 51st century. Specifically, Jack was born in 5094, 3071 years from now.
His original name was Javic Piotr Thane. Jack, as he later became known, lived on a colony planet with his family until an alien invasion attacked the planet and killed most of Jack's family. The only exception was Jack's lost brother Gray, who was tortured by aliens for years and finally came back to get revenge on Jack, which led to the death of Tosh and Owen in the second season of Torchwood. Jack knew loss once again when he was forced to kill his own grandson in order to stop an alien invasion.
3 There Are Multiple Versions Of Jack
Thanks to the power of time travel, there is more than one version of Captain Jack Harkness running around the Doctor Who and Torchwood universes. Other than the real Jack Harkness from whom Jack stole his name, Jack encountered his younger self at one point in an audio adventure while his younger version was still working for the Time Agency, and they had a fling together. Another time, there were two versions of Jack in Cardiff for more than a century, even though they didn't know about each other at that point.
After his brother Grey buried Jack alive 2000 years in the past, Torchwood found Jack in the early 20th century and froze him for a century so that Jack could return to his own time. Meanwhile, the younger version of Jack (the pre-burial one) was already working for Torchwood at the time. And during 1941, there were three different versions of Jack: the young one traveling with the Doctor, the one working for Torchwood, and finally, the one frozen inside Torchwood. If it sounds wibbly-wobbly-timey-wimey, as the Doctor so eloquently put it, it's because it is.
2 His Different Jobs
Other than being the Doctor's companion for a while and working for the Torchwood institute for more than a hundred years, Jack had several interesting jobs over the years. He worked for the Time Agency, of course, until he learned that they stole two years of his life. He lived as a priest in the 17th century for a while and used the name of Julian Horst, sticking to the initials he had previously chosen for himself.
He was a con artist before he met the Doctor. During a trip with the Doctor, Jack worked as an undercover reporter. And finally, his job as an enabler for the Committee on Kepri 5 led him outside of Earth but even then Jack tried to protect the planet from potentially hostile aliens.
1 The Details Behind His Creation
Just like all major characters of Doctor Who, Captain Jack Harkness could have been very different. For example, before the show's creators settled on his name, he could have also been called Jax. His surname Harkness comes from Marvel Comics, specifically from the villainous Agatha Harkness who also appeared in the MCU in WandaVision.
Jack could have had a different accent as well. Actor John Barrowman tried out several different accents during the audition process, including the Scottish and English accents. In the end, it was decided that Jack would speak with an American accent. And funnily enough, before being cast as Jack, Barrowman wanted to play the Ninth Doctor!