Q: Why do Doflamingo and Violet call each other “Doffy” and “Viola”?
Oda: Actually there is a profound setting about it, but I’m afraid I can’t tell you. I told my editor about it, though. Considering ONE PIECE is Shounen manga, I’d like to hide it since the reason is for adult audience. Dressrosa is a nation of passion, indeed!!
NOTE: I guess that, this is why she did, it’s because of making Doflamingo himself to spare her father’s life, the former king’s life, King Riku Doldo III (Who’s been the regular king again of Dressrosa).