A random thought that just came to me while watching Greeks Bearing Gifts: isn’t Torchwood Jack a bit like the Doctor?
They both have mysterious pasts that their companions don’t know about.
For example, the Time War dominates the Doctor’s recent-ish history, whereas with Jack, it’s being brought back to life by Bad Wolf.
They’re both alone, and set apart from everyone else by something unique to them.
With the Doctor it’s the whole thing about being the Last Time Lord: he will outlive everyone and no one can really understand him.
With Jack, it’s almost the same thing: he can’t die so can he really be said to be human? Instead of losing his race, he’s lost his identity as a member of that race.
These things that differentiate them also define their characters.
The Doctor, post Time War, is a much darker character than any pre Time War Doctor had the capacity to be. He still has his lighter side, but you can be reminded at any time that this is a man who has seen his entire race die, most probably at his own hand.
Torchwood Jack is a very different man to flirty con man Jack. The Jack who travelled with the Doctor still had a dark past, (Time Agency, memories stolen etc.) but he was a much lighter character on the surface. With Torchwood Jack, the darkness is always much closer to the surface.
In Torchwood, the Torchwood team play the role of companions. And I think Jack loves them in the same way the Doctor loves his companions. He loves them all, but not really in the sense we understand the word. His relationship with Ianto could be seen to parallel the Doctor’s relationship with Rose. There’s no doubt that he cares deeply for him, but he does have more important concerns.
I think the Tenth Doctor and Jack dynamic could be very interesting: it could be seen as mirroring the Ninth Doctor and Jack. With the Ninth Doctor and Jack, the Ninth Doctor is the man with the darker personality, who has not yet healed and Jack has a dark past, but has a lighter personality whereas with the Tenth Doctor and Jack, these positions may be reversed.
So, anyway, this might not make much sense but that’s what I thought.