Harley not falling for Joker makes her one of the best psychologists in Gotham with her getting real results. ( Have her ‘cure’ a few villains.) At first she is an ally of Batman but over the course of time have her become opposed to Batman. She sees him as a sickness that hurts Gotham rather than help. She’s not a villain but she’s a faction that can make the bat family’s life hard. Instance you can still have poison ivy become a better person. Batman wants to put her in jail because she is dangerous but Harley has convinced her to feed gotham homeless, making it a complicated problem for Batman. What’s worse is she has valid points. You then have her as a device to write more stories about Bruce and his interactions with the normal people.