Too much Dark Knight, I went with "Why So Serious" scenario for Joker.
A young boy was scampering down an a desolate alley way, panting hard. He looked around nervously, hoping he wasn't being followed. The boy sighed and started walking again. He'd lost track of how far he'd gone, and didn't really care, this damn city was bound to lead him back to where he care from sooner or later, and in one way or the other.
The boy heard a crash from up ahead, and wondered if he was already back at the last place he wanted to be, home. He ducked behind a dumpster and waited for the familiar noises of his parents' arguing; but they never came. Instead he heard a soft humming, not any recognizable tune, but a tune none the less. He peered around the side, and there was a girl walking down the alley way! She was smiling, and had a small bounce in her step, her red hair trailed behind her in gentle waves. She stopped at a dumpster a few ahead of his and looked inside. He saw her face light up.
"Aha, I knew there'd be one in here," she said, reaching her hand down inside of it.
She quickly produce a rose, that must've been discarded. She held it up and examined it then started playing with its satin petals. The boy deemed it safe, and cam out behind the dumpster. The girl saw him and smiled.
"Hi there, what's your name?" she asked bluntly.
"How can you act so calmly, in these times, its unsafe to trust some stranger off the street," he replied.
"That's why I'm asking your name, so we won't be strangers, we'll be...acquaintances," she said, touching the rose to her full lips.
The boy didn't respond, just stared at her in awe. The girl heaved a sigh.
"Honestly your too stiff, and much too serious, you should relax, see the funny side of things," she said, "By the way, my name's Aurora."
With that, she walked off, still clutching the rose close to her.
That was then, this was now. The young boy had grown, and was now one of Gotham's most feared criminals. The ever smiling, war-painted clown, the Joker. What happened to his "acquaintance?" He forgot all about her, and besides he had much more pressing matters at hand: like how to watch Gotham burn. He'd already sent his message out about wanting Batman's real identity, now he'd watch and wait, and crush some of Gotham's more important faces.
He was strolling down an alley, humming a tune in his head only he ever heard. He felt a crunch beneath his foot. Joker paused and picked his foot up, he'd stepped on a rose.
"Now who would leave something like this laying out here?" he asked picking it up.
"Hey sir, you gonna keep that beauty?" a voice called from above.
Joker looked up scanning the sky. Then someone jumped over the side rail on a fire escape a few floors up and landed behind him cat like.
"Down here," the person called, straightening out.
"Do have the slightest clue who you're messing with?" Joker asked them, getting annoyed.
"A dude in an alley thats probably gonna toss that flower out," they replied, in a childish voice.
Joker spun around on the spot, the only one aloud to play games was him. He was half way to pulling out a knife when he stopped, he recognized her, like she was from a dream.
"Have we met?" he asked her.
Joker could see a flicker of fear in her green eyes when she saw him, and he could see her breathing harder.
"I...i can't recall," she replied, tucking a strand of red hair behind her ear.
"Your the girl, from....the..uh alley, aren't you,"
She didn't reply just was inching ever so slightly to the left.
"Well, well, well, this is an interesting turn of fate," Joker replied, regaining his normal airy self.
She looked really scared now, and it was annoying to Joker. She was nervous, and antsy, realizing her old ways finally got her in trouble.
"Aw, you look nervous. Is it the scars?" he asked walking slowly towards.
It was either bravery, or fear, that kept her rooted to the spot.
"You wanna know how I got em?" Joker continued, thinking up a quick lie, and pulling out a knife.
"No," she replied, jumping to her left and hoping onto a drain pipe and climbing up.
"Your making this so much fun," Joker called, replacing the knife and pulling out a revolver. He took aim and purposely missed, she lost her grip and slid a bit, but recovered, and reached the roof. Joker merely laughed, he knew shed be forced to climb back down on the other side, and he also knew she knew it was hopeless, if it wasn't here, he'd catch her somewhere else. And his most vital clue, as laying limp in his other hand: a rose.