Tony Stark had always lived with guilt.
It was woven into the core of him—every invention that ended up in a war zone, every decision that put someone else in harm's way, every mistake he wrapped in a smirk and drowned in scotch. But nothing—nothing—crushed him more than the day he lost his little sister.
Kaley.
His best-kept secret. His soft spot. The one thing in the world that made him feel like more than just the arrogant heir to Stark Industries.
But there had always been something different about Kaley. Even as a child.
She was faster than she should've been. Quieter. Sharper. Her reflexes were uncanny, her instincts too quick. Doctors called her a prodigy—Tony called her weird and brilliant. Howard just called it "unreliable data."
She'd collapse some days with fevers no one could explain. Her eyes sometimes shimmered gold in dim light. Technology would spike or flicker when she got upset. Machines short-circuited in her presence if she cried.
Tony didn't understand it back then. He just knew she was special. Not just gifted. Altered.
Howard, though? He'd taken blood samples behind closed doors. Had whispered with old friends from SHIELD. Had locked Kaley out of whole wings of the mansion.
Tony hadn't realized it until later. Until after she was gone.
He'd been seventeen. Too young to carry a legacy. Too proud to ask for help. Too stupid to think the world wouldn't come for her.
It had happened fast. One moment, she was drawing with her tablet on the hotel balcony, hair tangled in the breeze, lost in her little world. The next, she was gone.
He remembered the nanny screaming. The overturned chair. The untouched glass of juice.
He remembered how the hotel staff looked at him like it was his fault—because it was.
The investigation spiraled into nothing. Swiss authorities shrugged. Local police buried it in paperwork. No ransom. No witness. No clue.
And the worst part? Tony felt it—deep in his bones—that whoever had taken her wasn't just some random predator. It had been clean. Precise. Professional.
Because deep down, he knew what his family name meant. Not just money and tech.
Power. Enemies.
The kind who didn't settle for hurting you. They hurt what you loved.
He tried to trace it back. Dug through every report he could hack. Cross-referenced travel manifests. Begged, bribed, and bullied contacts in embassies and newsrooms. But all he ever found were dead ends.
Howard had made plenty of enemies. But Tony? He made more. And with each unanswered call, with each blank folder, his obsession grew.
But years passed.
And with them, hope faded.
He buried himself in projects. In prototypes and circuits. In distraction. The world only knew him as the reckless genius. The billionaire playboy. But behind every joke, every explosive headline, there was silence. A gaping space where Kaley used to be.
He told himself she was gone.
He didn't know who had taken her. He didn't know why.
All he had were theories—each darker than the last. He paced boardrooms like war zones, lost sleep to surveillance footage, snapped at every press question about his family. Every dead lead felt like a personal failure.
He started pulling favors. Making calls behind closed doors. Using the Stark name like a crowbar to pry open records no seventeen-year-old should've touched. But nothing stuck. No ransom demand. No proof of life. No remains.
The world moved on. The press buried it. His father buried it deeper.
Howard called it a tragedy but treated it like a scandal. Stark Industries was under pressure, under watch. He didn't want distractions. Didn't want the company's name dragged through more headlines.
"Drop it, Tony," Howard had said one night, voice clipped over bourbon. "There's nothing left to find."
Tony didn't believe him. Not then. Not now.
So he worked around him. Accessed private logs, hired ghost consultants, bribed hackers on dark channels. It wasn't just about grief. It was about rage.
Because Howard had power. Resources. Influence. And he did nothing.
But Tony never did.
He kept her room the same.
And sometimes, when no one was around, he'd go in and sit by her desk—legs too long now to fit under it the way they used to when he'd help her cheat on math problems or build little bots from scrap parts. He'd pick up the drawings she left behind, half-sketched circuit boards and robots with cartoon eyes, and pretend for a second that she was just in the other room.
He remembered one day clearly. She was five, maybe six, and had managed to rig one of his workshop drones to follow her around like a puppy.
"Kaley," he'd groaned, half-impressed, half-panicked. "You hijacked my code."
She grinned—gap-toothed and triumphant. "You left it open."
"You're not supposed to know what any of that means."
She tilted her head. "You're not supposed to leave your schematics on the floor."
He laughed so hard he cried. She hugged him so hard she knocked the wind out of him.
He remembered another time—just a few months before the trip.
Kaley had wandered into the lab when she wasn't supposed to, oversized goggles slipping down her nose as she balanced a screwdriver in one hand and a half-melted toaster in the other.
"Tony, can I borrow your arc welder?"
He blinked at her. "That depends. Are you building a death ray in there?"
"No," she said, pausing. "But if I were, I'd tell you it was a toast-reheating laser so you wouldn't confiscate it."
He tried to look serious. Failed. "You're terrifying."
She grinned. "I get it from you."
He'd knelt down beside her, pretending to inspect the toaster. "You're gonna be something, you know that?"
"I already am."
And she was.
He remembered one more thing.
Kaley had been scared of thunderstorms. Not the lightning—she liked watching that. It was the sound. The sudden crack that rattled the windows. She used to sneak into his room during storms, barefoot and blanketed, and crawl into the space beneath his workbench where she said "the thunder couldn't find her."
Tony would pretend not to notice. Just casually slide a protein bar or a juice box down to her while he kept tinkering.
One night, when the storm was especially bad, she'd whispered, "You'll always find me, right? If I get lost?"
He'd looked down and nodded, heart in his throat.
"Yeah, shortstack. Always."
He never thought he'd have to prove it.
He updated her favorite tablet every year.
Even designed a prototype locator chip, despite knowing she never had one.
Because if she was alive...
Then someone out there had stolen the one thing that ever made him feel human.
And they would pay for it.