Doom Patrol season 1, episode 4, "Cult Patrol," introduces the team to wizard Willoughby Kipling, who seems curiously cut from the same cloth as John Constantine. With his chain-smoking, trenchcoat and mysterious manner, it would be all but impossible not to think of Constantine watching Kipling in action. This is by design, however and points to one of the odder occasions in the behind-the-scenes history of DC Comics.
The action of "Cult Patrol" centers around Kipling as he arrives at Caulder Manor in search of The Chief. It seems that a sinister society known as the Cult of the Unwritten Book are about to perform a ritual that could destroy all reality and Kipling had hoped his old friend Niles Caulder might be able to help stop them. The Doom Patrol are largely happy to help but soon find themselves put-off by Kipling's immorality, his slobbishness and his general strangeness and failure to explain exactly how his magic works.
This portrayal, and indeed much of the action of the episode, is taken directly from Doom Patrol #31 - the comic which introduced Willoughby Kipling and the Cult of the Unwritten Book into the DC Comics universe. When plotting the issue, Doom Patrol writer Grant Morrison had intended to use John Constantine for the story, thinking him a natural guest star to bring in to join with the Doom Patrol in battling some magical weirdness. Unfortunately, the editorial team of Hellblazer - Constantine's solo series - disagreed.
While DC Comics had yet to establish its Vertigo imprint for mature readers in 1990, there was still a policy of limiting contact between those characters such as John Constantine whose comics were aimed at older readers and DC's superheroes. It was also felt that Morrison's story in Doom Patrol was too fanciful to involve the relatively more grounded Constantine, who was written as a con-artist who played at having more magical talent than he did, rather than a blue-collar trickster mage. Thus Morrison created Willoughby Kipling, who was the more overtly magic-using John Constantine of Alan Moore's Swamp Thing in everything but name.
Though DC's editorial policy would change later and both Hellblazer and Doom Patrol would both go on to be part of the Vertigo line and a separate shared reality set apart from the main DC Universe, future writers would continue to use Willoughby Kipling even though the need for him had vanished. The Doom Patrol series version of the character, as played by Supernatural's Mark Sheppard, is both a tribute to the odd history of the source material and a worthy partner to Matt Ryan's take on Constantine. It remains to be seen, however, if either of these con-man magi will be showing up to plague Alec Holland in DC Universe's upcoming Swamp Thing series.