John Constantine and Swamp Thing have one of the most complex and long-standing friendships in the DC Universe. Constantine was first introduced in the Swamp Thing's solo series and although they don't get along flawlessly, the mystical heroes' fates seem to be intertwined. This was demonstrated once again following Brightest Day, when new version of John was introduced to the main DC Universe, after he had been isolated in a Vertigo series for decades.
Created by Alan Moore, Rick Veitch, and John Totleben, John Constantine first appeared in The Saga of the Swamp Thing #37 in 1985. Although Swamp Thing was often annoyed by John's sardonic personality and sly methods they nonetheless forged a partnership and John became an important supporting character in the series. He acted as a kind of supernatural mentor to Swamp Thing and helped him gain greater mastery over his powers. Swamp Thing even used John's body to have sex with Abby Arcane and conceive a child, although this was something of a betrayal as John wasn't fully willing to participate in the endeavor.
John became very popular and in 1988 received his own series, Hellblazer, the first volume of which ran until 2013. Starting in 1993, Hellblazer was published by DC's Vertigo imprint, which focused on adult content, allowing John's tales to become even darker and more sophisticated. The Vertigo series was only loosely connected to the wider DC Universe, and for a time DC's editor banned Constantine from appearing outside of Vertigo titles.
One of the unique aspects of the original Hellblazer series was that it allowed John to age in something close to real time (despite demon blood in his body slowing the aging process somewhat), unlike most mainstream comic characters who often stay around the same ages for decades at a time. Because of this, the original incarnation of John would be around 70 years old today. However, when John made his return to the main DC Universe in the final issue of Brightest Day in 2011, he was much younger than his Vertigo counterpart.
Soon after, the DC Universe was rebooted by Flashpoint, leading to The New 52, during which Constantine became a much more pivotal character to the universe as a whole. He was a main character in the first Justice League Dark series and he and that team have since become somewhat synonymous with one another. He also received a new solo series simply titled Constantine. While there are some who object to the character being used in the mainstream, it's indisputable that his return to the main DC Universe has given John more visibility, which can only be a good thing for the character.
Constantine has had similar success in group settings in other media. Both the live-action Constantine movie and television show received mostly negative reviews, although the former has gained a cult following. But Matt Ryan's comic-accurate take on the character from the latter was enthusiastically received by fans and incorporated into The CW's Arrowverse, where he has thrived as a main character on DC's Legends of Tomorrow for several years. The creators of the Arrowverse adaptation of Crisis on Infinite Earths, which Ryan's Constantine appeared in, have stated that they had hoped to include a meeting between Constantine and Swamp Thing in the event, and it may still happen eventually.
Ryan also voices the character in the Justice League Dark animated film and its sequel Justice League: Dark Apokolips War, as well as in a spin-off web series called Constantine: City of Demons. But there is another attempt at a solo adaptation on the horizon. J.J. Abrams' production company Bad Robot is in the process of developing a new live-action Constantine series for HBO Max, with a younger actor of color being sought for the title role.