This is what I heard about Harley Quinn as it was flowing through the Rumor Mill. Harley Quinn was supposedly at first the Joker's Psychiatrist and during the course of several elongated sessions at Arkham Asylum the unthinkable actually happens which is that she falls in love with him and at first he was not all that interested in her but eventually he falls for her too. Eventually they do get busy and get married and she gets him out of Arkham and they go on killing sprees together eventually the joker loses interest and they break up but she is never able to completely let go of all her feelings for him but she found after the killing sprees that she loves to kill as much as if not more then the joker so she continues to kill to this day and she also at times as an ex stalks the joker to this day. By the way unlike the Joker whose calling card is either a Joker playing card or something joker related Harley Quinn does not seem to leave a calling card.
Have you ever read Batman Adventures: Mad Love? It's a great graphic novel written by Paul Dini and drawn by Bruce Timm (both of them were co-creators on Batman the Animated Series), and it's all about Harley's origin. I think it was adapted into the animated series at some point, but it was a comic first. I honestly what she's like in the New 52, though. I haven't kept up with the Batman books.