I don't own Batman TAS, and I don't own Harley, Joker, or Batman.
Harley was curious. Who wasn't? As far as she knew, there were few people who knew who exactly was beneath that mask. From the obvious, she knew he had to be funded, which meant he was rich, or he was hired by someone rich. She figured whoever it was felt he had to repent, or that there was some tragic event in his life- a very traumatic one, she figured- that had shoved him over the line and into the territory of spandex and cape wearing.
Unfortunately, if he had lived his life in Gotham, these conditions weren't too narrow. There was a large community of overly-rich people who liked throwing money away for amusement, and who wouldn't think twice about funding some dressed up lunatic to fight crime. But the number of rich people who could be Batman? Those were less.
Harley closed her eyes. She had been waiting four hours for her Puddin' to get home from a heist she hadn't been invited to, and he still wasn't home. She hoped Batman hadn't gotten him. Ivy was in Arkham, too, so she'd have to steal alone were she to steal, and she hated that. She was a social kind of person, after all, even if those she stuck guns at might argue with this.
"Harley." She turned, and her eyes widened. Batman? He stood in the doorway, and she quickly looked down to make sure her robe was closed. Thank J, it was.
"At Arkham. Where he belongs." He answered, and she narrowed her eyes.
"In your professional opinion, am I right?" He simply glared, then said,
"Why? So you can stuff me in a room like the one my Puddin's in? So people can try to 'fix' me using the same stuff they taught me when I went to school?" Batman narrowed his eyes.
"No, you listen to me, Mr. Bat," Harley answered, angry. She had waited four hours for her clown, and now he was in Arkham. "Who exactly do you think you are?" Then she grinned, and said, "Or maybe you should just let me guess, since that's what I've been doing lately." Batman seemed unsure of what she was talking about, so she leaped in with, "What happened when you were younger, Batsy? Why do you think it was your fault?" His eyes widened- nearly imperceptibly- but Harley had been trained in this, and she could see his jaw tense. "Do you think stopping crooks like me and Mr. J will change what happened to you when you were a kid?" It was a longshot, of course, to say that it had been when he was a kid, but it was, really, only a fifty-fifty chance. He seemed about thirty- give or take a few years, maybe he's forty?- and being a kid, she figured in his mind, went to about twenty, or twenty-five. It did in hers'.
"Listen-" His voice was tense, and she knew she had been right. Whatever had happened, he had been younger then twenty. She guessed. This was all guessing, of course, for her. But for him, it was someone getting precariously close to a close-guarded secret. And the psychiatrist Quinzell in Harley was cheering her on, telling her to goad the dark knight until he cracked, cried on her shoulder, paid, and set up another session.
"Come on, Batsy, why should this- fighting us- help? Money didn't." The twitch, the unhidden fear. She was right; whoever it was, it was a rich person. She grinned, then said, "But, I guess it's your choice, B-man. Take me off." Oddly, he seemed set off by the nickname as well, so she simply smiled broadly.
"Quinn…" He murmured finally. "No one will believe you." She smirked, and she knew she had him. She didn't actually have him, of course, but she was so close, she could feel the tension rolling off of him in waves. So she did what Joker would do. She laughed, and then she said,
"I don't care, Batsy…it's just a joke. And a little secret for the Harleen in me to chew on." And, so while she had a little giggle at the Dark Knight's expense, the Dark Knight himself got a little hope from her last comment. That there was a little Harleen in her. Maybe she'd be saved one day. Or maybe she would just keep his secret to herself. Either way, he took her to Arkham, and Harley Quinn wasn't able to tell the story for a while…she just laughed.