Header was a bounty hunter and friend to John Constantine, created by writer Garth Ennis during his run on the title. By his association with Constantine, it seems that Header specialized in hunting down mystical items. Whether he took any other kind of contracts or whether he himself was a magic user of any sort was never determined.
In Ennis' final story arc on the title, Header is killed off early on; expiring from a massive wound to the abdomen; what looks to be a shotgun blast. Header bleeds out on a bus stop bench with Constantine beside him and the exact situation of how he was shot is never shown; the scene beginning with the two already on the bench and it isn't even clear at first that Header is injured (his stomach is covered by his coat and he only begins bleeding from the mouth a few panels in). Header's death, and those of other friends of Constantine's during this story, all of whom were created by Ennis during his run on the title, are the result of Ennis not liking having other writers who will be taking a book over once he's gone "play with his toys".
Header appears in a non-speaking capacity once or twice years later under different writers' care as members of the lineup of "ghosts of old friends" who occasionally appear to Constantine.