As you probably already know, Lysandre's name is based on the flower "Fleur-de-lis." I swapped out the "i" for a "y" for two reasons: one, having the "y" there helps people who aren't familiar with the Japanese version identify who the character is a bit easier, and two, because corrupting the spelling of the flower's name helps make the man's name seem a bit more like a person's name, if that makes any sense.
"Fleur-de-lys" is actually an accepted spelling of the flower name, and only slightly less common than the "lis" variant, though.