He woooullld get really upset and try to hide from him. He'd also make sure Kiritsugu was aware of him so he wouldn't get surprised! And then maybe ask him what they should do... He'd move in with someone else if he could too, for his own safety! But he'd be really unsure as to how it would affect Shirou, and so he'd probably try to hiss and puff at him to make him go away. Like an ineffectual alleycat.

What would a canon Kirei do meeting Byakuya, if you had one. :")