Note: This is total crack. The main idea [Spook and Nox being siblings raised in Underfall and stuff] was thought of right after TLO, and then Uma was added in. Of course, this doesn't fit TDP canon, so completely ignore it.
Disclaimer: You know already. No. Own. Do. I.
From the day she was born, Nox knew she was different.
The obvious difference was her race. Nox was human. Underfall was inhabited by darklings. With her pale skin, black hair, and piercing blue eyes... she stood out. And her height. Even at the age or five, being short for her age, she was as tall as nearly everyone else. By the age of ten, she towered over everyone.
Except Spook.
The only other human in Underfall, and her older brother... Nox couldn't imagine life without him. Even though he was seven years older than her, he kept close to her. Only her. Nox knew he didn't like people much, always slipped away into the woods surrounding Underfall whenever possible. Took all of his frustrations out on the ghouls and ghosts. Sometimes he would come home hurt, but he never cared. Not once did he ever stop.
Until, of course, the day he stopped breathing. But that was different. Later.
Nox never knew how she ended up in Underfall. She didn't know her parents. The darklings had been unwilling to take her in, until a pair very graciously accepted the two. Whenever she would ask, they'd just steer around the question. Turn the subject. They never gave her a straight answer. Always, "Did Spook leave again, Nox?" or "Are you still hungry, Nox?" or "Are you tired, Nox?" Never what she wanted.
She also never knew what was beyond Underfall. Never had she gone all the way through the woods. She wasn't allowed to. Spook would kill her if she did. He was like that. Always had been.
One day, though, when she was twelve years old, and he was being especially flighty... she asked him. "Spook, why are we here?"
He had been standing on the edge of the platforms, balancing on his toes, dark green eyes staring out at nothing. At first, Nox thought he hadn't heard her, so she prepared to ask again, and was promptly cut off. "Why do you want to know?" He threw a loose piece of gravel from the platform over the edge, and Nox solemnly watched it sink into the fog below.
"Because I want to know why we're here when we're human, and why we got taken in? The darklings wouldn't have accepted us if we weren't here for a reason." She fixed her eyes on him. "Spook, you obviously know something. Crowslip and Ugot don't suspect anything when you leave, but I know you're not just going out into the woods. It's suicidal out there."
Spook closed his eyes and leaned back. "Look, Nox, you're right. I do know... things. But I can't tell you."
"Why not? I'm twelve, and I'm better with a hatchet than you'll ever be. I'm strong, and I'm smart. You know that."
He placed his head in his hands. "It's more complicated than that. If I tell you before you're supposed to know, both of us will be turned into statues to be put in her garden. There are plenty of them." He sighed. "And even this is too much."
"Whose garden?" Nox asked, genuinely curious.
"Didn't you hear me? I. Can't. Tell. You." Spook turned and stormed off; approaching the woods he loved so much.
Nox stood back on the ledge, feel alone and embarrassed. She hadn't meant to put Spook in danger, she was just... curious. She always had been. That time was one of the many when she questioned if she and Spook were truly related. Sure, they looked similar enough... but shouldn't siblings act at least a little alike? Nox was open, sort of friendly, and curious. Spook was completely closed, angry, and serious all of the time. He absolutely hated being in crowded spaces with a lot of people. Nox liked noise. It calmed her down.
Angry, she pulled a chink of pavement and threw it as hard as she possibly could. And then, when it sank too far for her to be able to see it... she didn't hesitate to throw another. And another. And another. Until she had ruined about half of the ledge, but she didn't care. And then she turned back and silently walked home.