What Happened To Everyone Who Dated Joker?

When a person who appears to be Harley Quinn appears in Batman: White Knight, it's easy to presume that her Suicide Squad-esque appearance has no significance to the larger plot. However, this ends up being one of the most crucial details in the series, as it is later revealed that this sidekick is not the true Harley Quinn, but rather Marian Drews, who replaced Harley after the original, Harleen Quinzel, quit.

When Joker's experimental medicine restores him back to his sane self, Marian is outraged, having clearly been enamored with the Joker for his villainous antics more than the man himself. Committed to making Jack Napier back into the Joker, Drews takes his place as Gotham's biggest threat, and almost succeeds in destroying the entire city - only stopped when Napier and Batman put aside their rivalry in order to stop her for once and for all.

After risking both of their lives, the pair manage to defeat Marian, and lock her away safely in Arkham. There she safely remains - although Batman: Curse of the White Knight may see her return once more.