If you look up Viola's wiki page and go to the trivia section, you'll notice that it says that a certain fan once asked Oda, during an SBS, why Viola and Doflamingo called each other "Viola" and "Doffy". Oda replied that it was a deep secret, which couldn't be told, because it belongs to an adult part of the story."Viola", apart from being the name of some flower, means two things in Portuguese (we know that Oda likes to use different languages in his characters' names) "guitar" and "to rape". Is this the secret itself? Maybe Doffy did something to Viola and they grew some kind of affection for each other afterwards.
