Jack, in-between POTW and Last of the Time Lords with a little bit of Torchwood thrown in.
He keeps a lot of secrets from his team, wears his WWII coat like a shield. He wonders if leather really is better at keeping people out, or if the Doctor just has more practice.
He thinks this is what the Doctor feels. He finally understands the older man's reluctance to admit his feelings for Rose (for anyone, really). Because it hurts. So very much, to watch everyone you love die. He's been in love enough (lost so so many) but he loves the Doctor too. And the Doctor won't die (at least not anytime soon). He hopes (dreams wishes desperately hopes) the Doctor thought him dead, that the abandonment was not intentional. Because forcing him to live eternity watching everyone die…
That's why he takes so well to Gwen. She reminds him a little of Rose, reminds him of the human side of life. The compassion and love that's easy to forget when you live so long. The Doctor needs Rose (his heart stutters at the memory of a that name on the list of the dead and wonders how in hell the Doctor is coping, because if he finds it hard, it must be near impossible for the Time Lord) and Jack needs…well not quite Gwen, but someone, and for now she will do.
Perhaps this is kismet. The Universe getting back at him for being a con-man, for what he did during those two years he can't remember. Whatever he did, surely it's not worth centuries of Hell. Life-in-Death, with an albatross in the form of a broken vortex manipulator heavy on his wrist, his only respite, a hand bubbling in a glass case.
It's the only thing he has left of the only other person who can understand what his life (if this empty existence can be considered living) is like, and he waits. One day the Doctor will return and he will be ready.
Until then, he runs Torchwood. Fixes the broken agency and turns it into something better, and prepares the earth for the future.