It's almost without a doubt Poison Ivy. Tumblr is obsessed with shipping the two together.
Part of it is because, obviously, the Joker/Harley relationship is toxic as fuck in almost, if not all, iterations, but since Ivy has been described as one of the villainesses that Harley was "closest" to (no doubt because she treated her at Arkham before Joker got his hooks into her), they quickly went and rubbed their hot little hands over the interactions/friendship going "MINE MINE MINE LESBIANS LESBIANS LESBIANSSSSSSS"
...don't get me wrong, a Harley/Ivy relationship could make a great story, but not if it's done in the name of fetishism.