Senku had given him a head's up about Xue. This was the man who confessed that he'd killed someone on the train, before. Oh. Well. He already was playing cautiously, he certainly wouldn't stop now.

His expression remained unchanged, curious and a little amused. "That makes a good deal of sense - but I have to ask, as someone very new to all of this - why don't more people combine them, if they're so effective together?"