DC Database
The subject of this article has been mentioned only and never actually shown in a comic book or on-screen. Despite their lack of physical appearances, information about them might still be relevant or interesting or at least worthy of documentation. Anything known about them is to be considered second-hand information.

Harley Quinn may have been an ally of the Batman Who Laughs.

According to the Batman Who Laughs, he knew the Harley Quinn on his world, and the two became partners in crime. However, as pointed out by Raven, the Batman Who Laughs was trying to manipulate Quinn with his statement, calling the legitimacy into question.