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Ack.
Well, I'll be honest, there are people out there who can speak to this with hugely more authority and expertise than I can. I simply am not aware of all the nuance between these characters.
I know that Harley means the world to a great many female readers in particular. There's something in her and in that relationship that they find compelling. I'd love to hear from them, but I don't really have enough of a POV on her and Mr. J together to say. My first thought is that they are symbiotic, but others may disagree.
I do know that Harley done well is one of the best characters in comics, and Harley done badly is absolutely excrutiating!
Would love to hear from Harley fans in this situation.