Disclaimer: I own nothing associated with Joker/Harley.
Forget-Me-Not
Harley knew she didn't have the best memory in the world. Back in high school, she tried mnemonic devices, rhymes, even making up songs to go along with what she was studying, but nothing ever really stuck for long.
When she first joined The Joker, it was even more important to remember things, like not asking stupid questions or not trying to crack a joke of her own. After all, that was his job. When she slipped up, which was usually often, he never hesitated in letting her know. He had his own methods of teaching her, which, she had to admit, were quite effective. After spending more time with her Puddin', Harley was able to remember more and more, though she did occasionally make a mistake, but Mistah J was always there to help her remember.
It was because of Mistah J, Harley realized, that she had actually acquired a kind of photographic memory. There were certain things, for example, that she was very good at remembering, such as the vibrant green hair which would brush gently against her cheek while his fingers closed protectively against her throat. The deep, brown eyes which seemed to contain traces of violet as he firmly but calmly told her to leave his sight. His ruby-red lips, which would leave blood-red kisses along her throat, her neck, her shoulder blades, leaving a trail that would blend in perfectly with the purple and blue bruises dotting her arms and abdomen like forget-me-nots.
It was reminders like these which helped Harley most, because whenever she doubted anything or couldn't remember exactly what was expected of her, she only had to look at them like a cheat sheet, and she knew what to do. And at the end of the day, she was glad to have such a great tutor, because she knew her Puddin' cared about her deeply. He really did.
After all, he just wanted to help her remember. And, she added to herself, he was very good at it.