Jack got his start on Doctor Who as a companion in the first or 27th season, depending how you count. He was so popular that his character had his own adult-themed spin-off Torchwood. His character has his own comic books as well. Captain Jack Harkness is from the 51st century, and he’d been a time traveler long before he met up with the Doctor and Rose. He’s also a flirt. Jack’s charming and omnisexual.
Jack grew up on the Boeshane Peninsula in the 51st century. When Jack was a child, aliens attacked the peninsula. His father was killed and his brother was abducted. Jack became a time cop in order to save others from harm. However, when he discovered that the time cops had erased two years of his memories, he quit and became a time-traveling con man.
Jack first meets the Doctor and Rose during World War II. He saves Rose and brings her aboard his ship. Jack was selling spaceship parts to passing aliens, but he finds the idea of life with the Doctor and Rose far more appealing. (Plus, his ship’s blown up.) At one point, they travel to 200,100 and encounter the Daleks. Jack dies, but Rose using the TARDIS’ power brings him back to life. This makes Jack immortal. Literally every time he’s killed, he comes back to life. The Doctor is scared of what he’s allowed to happen and leaves Jack stranded.
Jack tries to follow them, but ends up in Cardiff 100 years before the Doctor’s schedule to show up. He joins Torchwood to help them fight alien invaders. As part of Torchwood, Jack did not always do good. In 1965, he gives gave 12 orphaned children to the 456 so they wouldn’t invade Earth. Jack becomes the default leader of Torchwood when the entire organization’s destroyed in Battle of Canary Wharf. He reformed the group to be the one seen on the Torchwood TV show.
In 2008, Jack finally finds the Doctor. When the TARDIS launches again, Jack grabs onto the outside and travels through time with it. His immortality allowing him to live. Along with the Doctor and Martha, Jack fights the Master. For one year, Jack is tortured by the Master and killed over and over. When the Doctor fixes everything, Jack is one of the few who remember what happened in the lost year.
When Jack heads back to his Torchwood team, they routinely stop alien and time threats on the time rift in Cardiff. He starts a relationship with one of his team members Ianto, and two of his other team members, Owen and Gwen, both harbor crushes on him at various points in the series. Torchwood is supposed to be Jack’s redemptive tale, about him going from a self-serving conman to a hero. He sacrifices himself, goes into traps knowingly to keep his team safe, and inspires loyalty from his team members.
When Jack’s brother Gray returns, he rips apart Jack’s team and kills Owen and Tosh. But Jack shows forgiveness to his brother, and instead of killing him (like he might’ve done even a year earlier), he cryo-freezes him. Torchwood continues with just three people, Jack, Gwen, and Ianto.
During “Children of Earth,” it’s revealed that Jack has a daughter Alice and grandson Steven. When the 456 come back, Jack loses Ianto when the British government tries to kill the 456 during negotiations. Jack, of course, survives. He ends up saving the children of Earth by sacrificing Steven. Jack is then seen leaving Earth, depressed and beaten down. Gwen waiting for him to return and lead Torchwood again.