Artwork is made by "CrazyCookieManic"

Charlotte and Luck could actually be half brother and sister.

First hint: Chala and Lufulu were siblings, and we learned that other elf's and their sibling were reincarnation with their actual human sibling like Gauche and Marie, we got Drowa and Elcat, so that means maybe Luck and Charlotte.
But Patri took over William's body, and Ratri (Patri's cousin) took over Langris body, but I can't find any lengths to them being related.

Second hint: This is Charlotte Grimorie 

Seems normal to be a grimorie of someone who use's planet magic right, but look at Luck's grimorie.

Noticed anything, for being a Lightning mage it's quite odd that there would be berries on his grimorie, but think about it, berries are related to flowers.

Third hint: Lucks mom has blue eyes and blonde hair and does Charlotte, there eyes are also about the same size.

Forth hint: We never see Lucks dad.