So, for 'Hellblazer'—wait, sorry, the original 'Constantine' books franchise technically started from Jamie Delano's run, right? Then it got huge under Garth Ennis, but I think the actual creator is Alan Moore? Actually, scratch that, John Constantine was created by Alan Moore and artists Steve Bissette and John Totleben, debuting in 'Swamp Thing' #37 back in 1985. But the long-running 'Hellblazer' series, which is the main book franchise, had its first writer be Jamie Delano. It gets fuzzy because a lot of people credit the whole vibe to Moore's original version in the Swamp Thing stories, where he was this seedy, chain-smoking occult con man from Liverpool.
What's interesting is that the 'Constantine' character became so much bigger than any one author, but if you're asking about the 'franchise' in terms of the books, the consistent narrative engine was the 300-issue 'Hellblazer' series published by Vertigo/DC. The real authorial voice that defined the character for decades, though, I'd argue is Jamie Delano. He set the tone—the political anger, the bleak British horror, the focus on consequences. Then Garth Ennis took it in a more nihilistic and darkly humorous direction, and Mike Carey later added this epic, tragic, almost mythic quality to John's personal hell. So it's more of a legacy than a single author.
Honestly, if you pick up the first trade paperback, 'Hellblazer: Original Sins', you can see Delano's fingerprints all over it. The franchise feels like a relay race where each writer added something indelible. But the core creation credit rightly goes to Alan Moore and the artists. I always liked that no one writer fully 'owns' him; it feels very Constantine to be a collaborative, messy, brilliant ghost in the machine.