It wasn't until his fifth year that Harry noticed her. He could never place what year she was - she looked younger and older than he all at once, her eyes large and round, her form dainty. There was age and wisdom set in the lines of her face, where there were any, but she was small and waifish, almost swallowed in her black school robes. And, whenever he caught a glimpse of her in the corridors, there was a Mona Lisa smile that turned up the corners of her mouth, before she disappeared in the throng of students.
He didn't know why his eye was drawn to her. She wasn't particularly pretty and didn't seem to be interesting. But whenever he asked his friends about her, they'd give him a blank stare, shrug, and say that they'd never noticed.
Still, with the events of the year, the niggling mystery of the girl was driven from his mind, more consumed with Umbridge, the Inquisitorial Squad, and keeping his friends and family alive. He didn't do a spectacular job on that last front, but nonetheless, he was sufficiently distracted that the mention of her in his sixth year threw him off guard.
It was Luna who mentioned her, the little Slytherin who spoke little and kept very much to herself. She was a fourth year, Luna said, and had fewer friends than she. "At least Luna wasn't Mara Darthe," her classmates would say, even as they bullied her. And suddenly, he had a name to match the girl's face, and her mystery lessened. The little French girl was just a little odd and quiet, that was all.
He didn't spare another moment on her that year. He thought he may have seen her at Dumbledore's funeral, but he'd been too overwhelmed to care. Too many things needed to be taken care of and it wasn't as if he knew the girl. But as far as he'd seen, she was the only Slytherin student to come.
When he saw her on the battlefield at the end of what should have been his seventh year, he took note. All Slytherins had walked out (or been kicked out) of the school not hours before, and there she was, flitting between duels and ducking under stray spells. And while he couldn't tell what side she was on or if she was even fighting, he knew, somehow, that she was there for support. Which was completely absurd, but he didn't have time to dwell on it, knowing he was about to meet death for what would likely be the last, and final, time.
But he didn't die. Well, he did, but he came back from that world in between, the one shaped in the image of King's Cross Station. And the pain that had been in his head for so long was finally gone, blessed relief and silence.
Harry never spied the little Slytherin after that; her body was never found and none heard from her again, if they even knew she was there to begin with.
And in King's Cross, Dumbledore held the crying, shriveled form of Tom Riddle, waiting for the train to come. He heard the whistle's squeal before he saw the smoke billowing from the scarlet engine, the shriek of the wheels as it came to a halt.
A girl of indeterminate age, with stylish dark hair and large dark eyes, stepped out and smiled her Mona Lisa smile.
"Hello, Albus," she said.
Dumbledore's eyes twinkled, but he said nothing. He held Riddle towards the girl and watched gratefully as she took the burden from his arms. She rocked the child-body and cooed at it reassuringly. Dumbledore thought he should feel surprised that Riddle calmed, but didn't.
"So," Dumbledore began. "Where are we going?"
The girl named Mara laughed, a raspy, breathy sound, and simply replied, "The next great adventure."