Ensuring an ex 'co-writer' can't claim infringement on my novel

So I'm currently in the process of writing a trilogy of novels which will be my first published works. I had a falling out with a friend who was helping out with plot and characters, specifically 2 characters he technically made himself, but has no real copyright over.

They have been severely unkind and venomous to me through the whole ordeal and have tried dragging me through the mud in our circles, and I think it would be fitting to have at least some revenge by writing their characters however I see fit.

But, I'm worried that if I should publish with any changes to their characters, this 'friend' may try to be petty and claim infringement with screenshots of our conversations about said plot and characters. Would they have any grounds without a copyright to those characters first, or would they pretty much be talking to a wall?