Capt. Jack Harkness is the biggest flirt of this and any other century, but who wonderes about that when knowing that he was a poster boy on his home planet, called The Face of Boe. He chats a lot without revealing too much - especially when it comes to his person. He was and is a man of action, just the cause behind his actions have changed a whole lot.
Capt. Jack Harkness has gone a long way - and you can see it not only in the clothes he wears, but also in his eyes, his words and his actions.
His fashion draws attention where ever he goes, at least as much as his charm, self confidence and looks. Because it is not usual to run around in 1940s-vogue, when you live in the 21st century and have been the first one to sign up to be a Time Agent in the 51st century, provided with special pheromone, an open mind and a wisecrack ability that must have even amazed The Doctor.
He's a natural leader for his team which he loves and cares about. His team follows him, but not without challenging him or questioning him, his actions and his past. He's a father-figure, his team can look up to, but at the same time himself searching and waiting for someone to rescue him. And waiting he's been for a long time - since he became immortal after the battle against the DALEK, when Rose brought him back to life with the help of the time vortex - and then she left with The Doctor.
Travelling with The Doctor, Jack started to become the hero he is as the TORCHWOOD THREE leader of Cardiff.
Being on Earth as a Time Agent, he lost 2 years of his memory and became a con man, taking the name of a deceased soldier he looked up to and causing a plague was only the beginning of a long journey. Being immortal, coming back to earth in 1869 with the help of his vortex manipulator and settling down to wait for the 'right kind of Doctor' had an even deeper impact on Capt. Jack Harkness and this planet. Taking the long way from being forced into TORCHWOOD as a freelance to becoming a TORCHWOOD leader himself, who changes the whole policy of the British institution.
They are not any longer only plain alien hunters - TORCHWOOD also helps aliens and earth to prepare for what may come in future and Capt. Jack Harkness leads the way. Sometimes quite merciless, but always with the ability to see the bigger picture.
For references, facts and analysis please check the very cool TW-Wiki-Info.