Alan Moore has stated that in an interview: Steve Bissette and John Totleben, the artists drawing the Swamp Thing (that Alan Moore was writing at the time) sent him notes, expressing a desire to draw a character looking like Sting.
So he created the character John Constantine:
But I can state categorically that the character only existed because Steve and John wanted to do a character that looked like Sting. Having been given that challenge, how could I fit Sting into Swamp Thing ? I have an idea that most of the mystics in comics are generally older people, very austere, very proper, very middle class in a lot of ways. They are not at all functional on the street. It struck me that it might be interesting for once to do an almost blue-collar warlock. Somebody who was streetwise, working class, and from a different background than the standard run of comic book mystics. Constantine started to grow out of that.