Ow, he's blind, not really but he definitely sees spots for a moment. No big deal, though, especially since Koushirou did the same thing to Ellie shortly after arriving, so he really can't judge.

So he just blinks hard a few times while William explains, but despite the distraction the response is simple enough that he doesn't have to do much mental translating and therefore gets pretty much everything said to him. Before answering, though, he steps closer to the tomb and reaches out to touch it, using the light from William's flashlight to look it over.

"I'm fine, thank you." It isn't flippant at all, completely genuine--he appreciates the concern but it's unnecessary--and quietly certain. This situation is weird and fascinating and probably a little dangerous, but he's been in worse. So far.

"Why do you want to open it?" That's a more interesting topic of discussion, and it's curious rather than judgemental; part of Koushirou is not super into the idea of opening a casket, for several reasons, but the other part of him definitely wants to find out what's inside. Everything that's going on right now has to be happening for a reason, right? And exploring everything involved is the best way to figure out what that reason might be.