This has no spoilers in it. Based off of the trailers.
This about You and the Joker. Ever since that night that you saw him race down the street it was then that your fate changed. when he saw you he knew he wanted you for himself. He was sick of Harley anyway. He wanted a new playtoy, and your were just right.
He watched your every move after that not that you knew. He waited for the right moment to snatch you for his own selfish self.