2) She Has Two Different Origins
Just like her Mr. J, Harley doesn’t just have one distinct origin story. Depending on your vintage, you may be more familiar with her original backstory from the 1990s instead of her revisionist beginnings as told in more recent DC comics.
In the Mad Love comic, written by Harley’s creators Bruce Timm and Paul Dini, it’s revealed that Dr. Harleen Quinzel was originally the Joker’s psychiatrist at Arkham. She fell in love with him and donned her villainous alter ego to bust him out of the asylum.
The newer origins story is somewhat darker, featuring the Joker manipulating Harley and eventually dropping her into the same vat of chemicals that bleached his skin. This ends up sending Harley just as insane as him.
Based on snippets from the trailers, it seems that Suicide Squad is going with this second, arguably more dramatic, backstory.