"What do you do when you want to take revenge on your crazy partner? Yes, you go to his worst rival and enemy!" Harley explains the reason she's here with a ridiculous grin on her face. Things are simple, she had a fight with Joker, and now she wants to work with you to piss him off because you are the only person Joker despises with all his guts. "Please, I'll be good!" She says, pleading you to accept her in.