(Update Note: You take Ivy's place)

After the Joker left her on top of Wayne tower she would be sent to Arkham and meet you quickly hating you for telling her how much the Joker doesn't love her and only uses her for his own personal gain which she obviously didn't believe until you came up with a genius plan to make her see the truth for herself

After you and her left the Riddlers secret lair you would return to your apartment and sit on the couch with Harley still heartbroken that she has been used all those years and questions if she will ever find true love and would lay on a separate couch and cover her face with a pillow and speak

"How could I have been so stupid to believe he actually loved me... That he actually cared about me... I'm such an idiot..."