Jack Hawksmoor is a member of Warren Ellis' comic-book superhero team, The Authority, having put in many years of Black Ops work as a Stormwatch officer. Each member of The Authority can be seen, to a greater or lesser extent, as the 'spirit' or something-or-another, and Jack is no exception: he is like the spirit of cities.
All of this is no accident. In his youth, he was repeatedly abducted, and operated on, by aliens. These aliens - from the future rather than outer space - systematically replaced Jack's organs with their own technology in order to make him the ultimate city-dweller. This wasn't just for kicks, either. The Kansas City of the future had gone rogue and destroyed the world in the 70th century, and it had time-travelling plans to do the same to our world. The aliens' intervention enabled Jack to sort this business out, but it was a lot for the lad to go through.
In practical terms, as long as Jack is in a city of some kind, he melts into city pavements, can order buildings and other architecture to fall or otherwise assist him, and feels a great disturbance in the force when a building or city is destroyed. Also, he seems to punch and kick with falling-masonry force, smashing heads and stuff right, left and centre. He draws nutrition from air pollution rather than food, and never needs to sleep. All cities are as one city to Jack, so he can travel between them really quickly.
Outside of a city, however, he can barely survive long enough to read a paper. They can keep him going OK on the Carrier, the Authority's shiftship, though - technology, don'cha know.
Physically, he wears a dark suit with no tie and a collarless white shirt, and no shoes. The shoeless thing has to do with these cool grippy soles that his feet have, kind of like in that Pirelli advert a while back. Also, his face is a little too small for his facial area, if you believe Frank Quitely.
Personality-wise, he's casual and calm, but really violent in fights. He is as dedicated as the rest of the team members to their cause (benevolent dictatorship of the world). He took over the leadership of the group after the death of Jenny Sparks, and continued in the idiom that she had established.
The name 'Hawksmoor' is almost certainly a nod towards Nicholas Hawksmoor, the obelisk-happy architect featured heavily in Alan Moore's From Hell, in which Moore details the secret lines of power that can be inferred from Hawksmoor's work in London. In fact, I'm pretty sure there was some talk of a 'Jack' in that book too ...
Jack Hawksmoor first appeared in Stormwatch #37, published by DC Comics' Wildstorm group.
for pre-Authority info.