Hey, I wish you good luck finding this, as you said, it's rather specific. It's usually a better idea to pre-plan RP with a close friend when it comes to such specific RP, however, I do not see it as an impossibility.
If nobody responds, you can also make an alt to play the antagonist yourself and have interactions between them happen off-screen, that's what I did, anyway. This can potentially open more doors for people who want to play antagonists who don't quite match up to your description, but would do something if his cousin were to hire someone. I know a myriad of players play antagonistic characters, so it wouldn't be unrealistic to treat it as if they were hired to cause trouble.
Just my two cents, good luck.
If nobody responds, you can also make an alt to play the antagonist yourself and have interactions between them happen off-screen, that's what I did, anyway. This can potentially open more doors for people who want to play antagonists who don't quite match up to your description, but would do something if his cousin were to hire someone. I know a myriad of players play antagonistic characters, so it wouldn't be unrealistic to treat it as if they were hired to cause trouble.
Just my two cents, good luck.