Judge Gallows:

I'd say if ANYONE was Cain's enemy it's this guy! One day Abel made a deal with Gallows to try Cain for always murdering him, in exchange for Gallow's freedom. That was rather disastrous, but in the end Cain beat him out. Believe me, Judge Gallows is atrociously annoying!

Lucifer:

Well, this is sort of an odd, all encompassing-type enemy. However, despite Cain having the Mark of Cain and all, I've never seen anyone put so much fear into him, or treat him so rudely. I personally would blame this whole ordeal on Lord Morpheus, who cruelly sent Cain out on a little errand to Hell.