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.