Lord Ren?
Master?
Neither of those really work for the specific lines we’re doing. In the first, they’re referring to Rey as “Kylo’s pet scavenger”. I guess “Lord Ren” sort of works here, but I personally like the idea of them being frustrated with him and his obsession with Rey, and not using an honorific title.
The other one is when they’re about to fight, so they definitely wouldn’t call him “Master” or “Lord”.