John Constantine is a
royal bastard and sometime
hero of the
DC Comics universe. First introduced by
Alan Moore in
Swamp Thing, he later got his own book,
Hellblazer. He's appeared in a few other
Vertigo titles, notably
Neil Gaiman's
Sandman and
The Books of Magic.
Constantine is an odd character -- a powerful magus from an English working class background, and with the appropriate sympathies for Labour and lager. He specializes in summoning demons, and he has an unfortunate history of getting his old friends killed by said demons. He wavers between the hero and the antihero in more or less every issue.
Most of the time, both Heaven and Hell are rather extremely pissed off at Constantine, and most of the time neither one can do bugger all about it.