The man himself - doubt dictates that Booker still doesn't believe her even after the confirmation. Lucifer? Actually existing? That thought has a ripple effect that Booker isn't keen on processing.

Well, maybe it's just a nickname for someone. And inside joke. Yeah, that's more likely.

The trouble part is a given but Booker nods his head in sympathetic consternation anyways.

"The world's your mollusks. Some places have got to be easier than others."