He would attack on sight/sound of him. No matter what X!Kamui and Fuuma told him. XD The hate, it is strong. And since he can't accept the idea of any Subaru loving any Seishirou, he would figure he's doing other Subaru favors by trying to get rid of the Seishirou.
He would also get his ass roundly handed to him, since all Seishirou are more powerful than he is. TRC!Seishirou would probably be the closest to him in ability, but Kamui would fail. And then he'd stand around and let people get mad at him for it without offering to explain himself.
I... am... kind of out of questions for now. But I'm also tired. @__@ I will get back to you on the next question. X3