Since its the Arkham canon, Harleen was a boring mid quality therapist who was so lonely she accidentally took Joker's "confession" to finding his purpose and love in life after meeting Batman to be about her, and then obsessively followed him since due to a crippling codepence to a point where she would fuck even a wrinkled old man who had some of his blood in him.

Which is also probably a story they should think about.