Doom Patrol comics writer Grant Morrison created Willoughby Kipling in 1990 specifically because he wasn’t allowed to use Constantine in his DC superhero comic, according to CBR.
Constantine was at that time generally relegated to only appear to comics from Vertigo.
If that sounds an awful lot like a description of John Constantine, there’s a good reason for it.
In the years since, Constantine has become much more closely tied to DC superheroes, and a version of the character is even part of the Arrow-verse on The CW.