Thanks to everyone who responded to my query, I now know that Grantaire is known as "R" because the letter "r" is pronounced "aire" in French and to say a capital letter you say "grand" before the letter (which I should have known, as I am studying pre-tertiary French this year). For example, a capital r ("R") would be pronounced "Grand aire", which is how Grantaire should be correctly pronounced. So essentially, Victor Hugo enjoyed puns tremendously and took great enjoyment from Grantaire signing himself simply "R" throughout the book.
Thank you all for putting up with my shit ♥









