This has all come on rather suddenly, but he remains calm. Still, this is a lot to ask of a mere stranger. A stranger with status, yes, but--

"On that night, I will watch your room. Should this person," and if the key came with the Comtesse, he suspects it is her, "come for the key on that night, I will deal with them." Deal with means either apprehending them on the spot or following them, he's not sure yet.

"Take a deep breath and tell me of your sister."