I like to write Copy Rezo as having some aversion to being touched, and I think it’d be interesting to try incorporating it into a sex scene. The setup I’m imagining is that he’d have your character in bondage so that they can’t touch him, and he’d be using sex toys on your character to keep the physical contact to a minimum. (e.g. prodding their erogenous areas with a vibrator instead of his hands, using a dildo for penetration.)

Canonically Copy Rezo does sprout some Secret Hand Mouths at one point and it’s not implausible that due to dark magic shenanigans he could wind up with more weird traits. I’m a simple person so tentacles are what I’m mainly picturing here but I’m happy to experiment with whatever other monster traits we can come up with. Some other suggestions: French kissing and oral sex with the hand-mouths, hemipenes, barbed dicks, claws and fangs, eyes in weird places, multiple limbs, inhumanly long prehensile tongue...

I just want him to shove his cock down someone’s throat repeatedly, that’s pretty much all there is to it.

Your character gets banged by Copy Rezo and the original Rezo. Due to the contentious relationship between Copy Rezo and his progenitor, you can have your choice of “a Lot of assumed CR happened before the threesome,” having them doped up enough on aphrodisiacs/magic/sex pollen/ABO-esq pheromones etc, or this maybe being a Fuck Or Die scenario. I’ll default to having Copy Rezo and Rezo focused on your character, but if you want your character to watch them bang I am amenable to that too.