Let's talk about names, actually!

Everyone in Foyle's world has a given, a surname from the town where they're from, and a tattoo/crest for their specific family unit (Foyle and Rook are cousins so their family names are ALMOST but not quite the same). By contrast Monday follows Western European convention of given name/surname being distinct for her family.

That's the in world logic, anyway! On the writing level:

Foyle's name was inspired by "foil" for fencing and gold leaf. Rook's was inspired by the chess piece before I realized it was for a bird and would be even more perfect. And Monday's name was inspired by an actual real life coworker of mine named Wednesday :)