SPOILER WARNING: This following article contains spoilers for Batman: White Knight #4 written and illustrated by Sean Murphy, in stores now.
The DC fandom was dealt a collective blow when Batman: White Knight revealed that there was in actuality not one but two Harley Quinns running around in Gotham City. There was the real, original Harley Quinn -- the one who wore the classic black and red court jester costume -- and a second, fake Harley Quinn, who wore the modern Suicide Squad movie-inspired red and blue costume. The first Harley had left and the second had taken her place, the Joker never once realizing the switch.
The differences between the two Harleys are stark. The real Harley is in love with Jack Napier, the man behind the clown mask, and she is truly supportive of his change of heart. She is a psychiatrist who understands him, one who didn't hesitate to leave when she realized that Joker's obsession with Batman was something she couldn't compete with. For her part, the fake Harley is in love with the Joker. She doesn't want him to get better -- she only wants him to return. And to do so, she adopted a new identity: Neo Joker.
In Batman: White Knight #4, we finally found out when the switch between the two occurred, and how the woman named Marian Drews became the second Harley Quinn.
Marian met the Joker after the first Harley had already left him. He robbed a bank where she worked as a teller. But when the Joker put a gun to her head and threatened her life, he had no idea that he was actually saving her. Earlier, Marian had slit her wrists, waiting to bleed out as she served client after client. But the threat of the Joker made Marian realize that, more than anything, she wanted to live. Therefore, in order to stay alive, she helped the criminal rob the bank.
Much to her surprise however, the Joker, in his insanity, started calling her Harley. Even she managed to see the resemblance, so she played along, acting as the real Harley would, constantly in the hope that he wouldn't kill her. And it worked a little too nicely because, soon after, he gave her one of Harley's old costumes.
He bandaged her wounds up, and took care of her. He gave her a reason to live, and a reason to be happy. For those reasons, she simply didn't want to leave him, or tell him that he had the wrong person. Instead, she became Harley Quinn, albeit a different version -- her own version. Even now, Marian recognizes that this is likely a case of Stockholm syndrome, but she simply doesn't care because the Joker gave her happiness.
Now, with the Joker gone and Napier in his stead, Marian has lost the source of her happiness. Now, she is forced to adopt a new villainous identity in the hopes of drawing the Joker back out -- and with him, her reason to smile once more.