I think this girl is/was a slave of celestial dragons and the (dead ?) fiancee/wife of Tesoro :
Indeed, the theme of celestial dragons seems to return often in the movie :
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…
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