I think this girl is/was a slave of celestial dragons and the (dead ?) fiancee/wife of Tesoro :
Something is missing… A wedding ring ?
Indeed, the theme of celestial dragons seems to return often in the movie :
Does the straw hats know the truth ?!
I think Tesoro wanted to bring back his fiancee/wife with his wealth, so he asked for the help of Doflamingo in exchange for a great deal of money :
Here, Doflamingo is clearly shown in a upper position, he probably served as an intermediate between the celestial dragons and Tesoro. However, it seems that the negotiations failed…
Does his evil comes from this story of his past ?!
I also think that his henchmen are former slaves on which Tesoro has replaced the seal of celestial dragons by its own seal : a star (as the fishmen with their sun symbol)
Money can’t buy happiness. It would seems that it is the theme of the next movie One Piece.
*Theory by Tear