She's the only person who's managed to become intimate on such a long term basis with the Joker, who, in turn, displays occasional moments of confusion and discomfort which results in attempts to kill her.
Her origin story relates that she met the Joker while working as a psychiatrist at Arkham Asylum, where the Joker was a patient. Harley Quinn is depicted as having a very pronounced New York accent.
Love your perfume. What is that, Stench of Death?