Sit down, grampa's got some learnin' for ya'.
Back in the old days when comics were drawn on leaves using the charcoal sticks from last night's fire, a big, scary-ass british man name Alan Moore created a very minor character to use as a plot device in his Swamp Thing revisioning. See, the Swamp Thing had never quite hit it off as a tragic superhero and had languished on the back shelves for literally decades, but Alan Moore figured with his powers and his connection to nature, Swampy might be good for much mor that one-off teamups with batman and Superman to fight Solomon grundy and Killer Croc.
To this end, he dragged the Swamp Thing into DC's rich magicverse (an insanely expansive universe of characters and settings brought on, for the most part, by DC's purchas of EC Comics back in the 60's), he added a bit of Islington new age crap and made the Swamp Thing into a force to be reckoned with. Throw in Moore's attitude toward sex in comics at the time (plenty, often, and exlicit) and Swamp Thing became one of the first truly successful title direct-distributed to comics shaps (magazine bundlers used by book stores and other mainstream venues wouldn't touch anything not bearing a CCA labelat the time).
But how to keep Swampy in the fray? Historically, the Swamo Thing was not a world traveller, and the odds against heavy hitters like Lucifer, Jason Blood, and Doctor Fate just wandering into a small bayoui town southeats of New Orleans are astronomical. So, Moore introduced John Constantine. Constantine, in Moore's books, was just a plot device. He popped out of nowhere, tricked or convinced the Swamp Thing into some mission or conflict, then disappeared.
At some point Garth Ennis took a long look at Moore's creation, and decided that Constantine was exactly what Great Britatin needed. The Punk revolution was over and the nation was in the grips of the Thatcher Recovery (which, if I remember involved putting people to work by beating them over the head if they didn't accept the low salaries and crappy benefits offered). Ennis liked the idea of a guy who was involved but not engaged, who made the wheels turn, then stepped back to watch.
He got permission from Moore (I assume) to use his character (even though, due to creator contracts in the comics world at the time, Constantine was technically a DC property), and pitched the idea to DC. Along with a long list of artists, Enns turned COnstantine into exactly that person, with the addendum that he never quite pulled back any more, he always seemed to get his coat-tails caught in his own devices.
Despite a lot of splashing around in a lot of harmonic convergence crap and bonghit philosophy, Ennis made Constantine into a bizarrely successful character. You couldn't help liking him and rooting for him a little, even if you knew in your heart that if you ever knew a guy like him for real, you'd have to punch him, early and often. He went on to create or revolutionize a number of DC's magic and creepy characters, putting dirt under their fingernails and making them seem less off and more, "just blokes". He is, and I say this without any regrest, one of the most undersung writers of the Vertigo revolution, and you should be thankful he exists (if only because his work provided the world with another reason to revile Keanu Reeves).
What I lack in sincerity, I make up for in sarcasm.