This is a great question! For those who may not know, Rook's nickname for Jamil is actually "Monsieur Multi," not "Monsieur Pyramide" as seen on NA.

The reason for his original nickname possibly comes from the multiple sides of his personality, as Rook is complimenting the "smoldering soul" inside of his "cool exterior" in the same sentence.

The original word used, マルチ, is quite literally the English-language word "multi," just written in katakana instead of romaji.

As for why NA decided to change this to "pyramide," as with most changes on NA, we do not know!

It is possible that the thought process was "multiple sides -> what has multiple sides -> pyramids?," but it may always be a mystery :<