Viola is one of the main supporting characters in the Dressrosa arc AND YET she doesn't get the focus and attention she deserves from the fans or the official avenues. imo.
Like I am a Rebecca fan and defender as well (not of her outfit tho), but every time there's official art of The One Piece Girls and there's Rebecca but no Viola it hurts me deeply.
Anyway I love Viola for her strength and bravery in the face of Doflamingo and her love for her family. And her crazy backstory.
One of the most compelling parts of Viola's story is her time undercover as part of the Donquixote Family. To spare her father's life she joined her enemy, Doflamingo. She lived 10 years with them!! She and Doflamingo were in a relationship at one point! And it makes you wonder, were those friendships real? Did she actually feel anything for Doflamingo or was it forced?
And to add to the tragedy of her time with the Donquixote Family, her power allows her to see into other people's minds. Even if she did start to care for Doflamingo, against her own best interest, she would be able to look and see that he would never be anything but evil.
But this provides a good parallel to Sanji, one of the Straw Hats she is closest to. When she betrays him on behalf of Doflamingo, Sanji isn't upset with her. When she looks into his mind it's nothing but the usual Sanji thoughts.