There's this theory that Cain is Zestial due to the fact that he's the oldest Overlord who's considered ancient by even Alastor, makes Sinners panic upon sight, and a couple theorists are saying his hat resembles ones worn by farmers from older times, which was Cain's occupation.
I'm neutral on whether this theory turns out to be true or if it even makes sense to begin with. Given how many characters this show already has, it'll make things easier to have Cain be incorporated as a preexisting character rather than be a brand new one, and it could expand on Zestial beyond "retired legend" and "Carmilla's boo".
Reasons those who don't think this theory holds water give include how unmarketable it'll be for one of Adam's sons to look nothing like the other, the lack of buildup, and the absurdity of it all, especially if they do go with "Rosie=Lilith, Lilith=Eve".
And I guess Zestial has a last name apparently. Link above is the only source of it. It's Morde, German for murders.