"Hiro, this is not a good idea."

"Oh come on, Tadashi," Hiro complained. "These drawings probably won't even do anything."

With his back pressed against the wall and his arms crossed, Tadashi continued to send waves of disapproval towards his little brother. Meanwhile, Hiro just continued to draw the Transmutation Circle.

A few weeks ago, they'd gone to the library together, Tadashi for his research project and Hiro for books on Advanced Robotics. He didn't actually need the book, since he'd practically memorized it when he was seven, but he was working on a new battle bot and there was something about magnetism that he wanted to look up. Robots required precision work. Not even a genius could work with half-remembered information from half a life ago. Well, Hiro probably could. He just didn't want to.

It was when he was looking in the robotics section that he found the tome on Alchemy. Specifically, Human Transmutation.

Tadashi had dismissed it as interesting nonsense, but Hiro saw a new puzzle for him to solve.

It took him two weeks to decode the book and another week and a half for him to gather the ingredients, all without ever telling Aunt Cass what he was up to.

While he worked on his "voodoo" as Tadashi had put it, Tadashi worked on smoothing out Baymax's kinks.

Both finished at around the same.

Which was why Tadashi was around and paying attention when Hiro sliced his thumb open.

"Idiot" he hissed, surging forward. "What are you doing?"

"Stop!" Hiro shouted before his brother could smudge the circle. "Blood is required."

"For what?" Tadashi asked, frustration leaking into his voice. His worry about this whole thing was eating him up inside and just standing around, not helping, was killing him.

But what bothered him most was that he didn't know why Hiro was willing to go so far as to injure himself if this was all just a game to him.

Hiro paused. Another drop of blood fell his hand. Tadashi tracked it with his eyes until it crashed into the cement and splattered.

"For mom and dad." Inhale. Exhale. "The book said it could bring the dead back. I didn't tell you because I knew you'd try and stop me."

What ever answer he'd been expecting, that hadn't been it. The shock faded quickly, but the worry didn't. Tadashi frowned, crinkling his brow as he pondered over whether he should stop this. A circle drawn with chalk and ash wasn't going to bring their parents back, but was trying really such a bad thing?

Maybe.

Maybe not.

He took one look at the hopeful and hesitant expression his little brother was making and sighed. "Give me the knife, Hiro. It'll work better with both of us."

Hiro's face lit up with his thousand watt smile, leaving his brother feeling helpless.

One slight pinch in his finger later and he knew there was no turning back.

The light show was unexpected.

At first, Tadashi chalked it up to chemicals Hiro hadn't told him about reacting, but then he saw Hiro's leg - shoes, pants, everything - break into pixel shaped pieces and dissolve into thin air.

He groped for Hiro's hand, only to see Hiro staring at him in wide eyed horror and realize that his entire body was dissolving.

Shadows, writhing and alive, sprang from the ground, pulling him away. He screamed and jerked, fighting to get to his brother.

"Tadashi!" Hiro screamed.

"Run!" There wasn't any saving him, the tendrils were taking his face. In a few seconds, there would be nothing left. But Hiro still had a chance.

Still, when he saw Hiro crawl over to him, dragging his body as far as he could even as he bled out, Tadashi couldn't help himself.

The last thing he saw was his own hand reaching for his little brother.

"No." Hiro said the word, he mouthed it, screamed it, whispered it.

And then he stopped.

Propped in the corner was the deactivated Baymax and Hiro had an idea. It was crazy, borderline suicidal, but at the time so was he.

Gritting his teeth, he dragged himself to the robot, leaving a crimson trail behind every labored heave he made.

Without anything else to use, he drew the seal on Baymax's chest with his own blood and then made his way back to the outer edges of the circle with as much speed as he could manage with one leg.

"Tadashi" he ground out, "is the only brother I have."

He clapped his hands.

"Don't take my brother away from me!"

And slammed them down on the transmutation circle.

Relief at still being alive flooded Tadashi the next time he opened his eyes.

He'd thought he was a goner for sure.

Note to self: Do not let Hiro play with old books in the future.

And speaking of Hiro, where was he?

Images of snake-like shadows rising from the ground and erasing his brother's leg jolted him out of whatever half-awake state he'd been in.

The room looked like the set of a horror movie. Though the transmutation was now nothing more than a dull outline on the floor, dark blood stained its edges. Tadashi tracked it with his eyes until they landed on the softly shaking form of his brother.

He ran forward, noticed distantly that his body felt unusually bulky, and kneeled at Hiro's side. When he reached out to cradle him, he saw the white arms, Baymax's white arms, and nearly blanked.

He was Baymax?

How had that happened?

Weakly, Hiro placed his hand on one of his thick fingers and cried, "I'm so sorry, Tadashi. This was the only way I could think of to save you. I'm so sorry."

"Hiro" Tadashi choked out, his gaze locked on Hiro's bloody sleeve. "Your arm…" There was nothing there. Just like his leg.

Instead of answering, Hiro's head lolled to the side. The pained expression never left his features, but his already deathly pale face seemed to blanch further and his eyes clouded over, shock finally claiming him.

"Hey" Tadashi shook him, barely even registering the quality to his own voice. "It's okay, Hiro. It's alright. I'm here. But you have to stay awake, okay? You have to stay with me."

Carefully, he cushioned his brother in his arms and gently lifted him off the ground. Without any of his brother's usual resistance, or even his usual weight, the action felt disturbingly like lifting a broken doll.

"Aunt Cass!" He screamed, hoping she could hear him. "Aunt Cass, call 911! Hiro's hurt!"

Everything else could wait.

Hiro was his one and only little brother.

He was stupid and reckless and precious and irreplaceable.

And Tadashi was going to save him.

AN: This was written for a prompt I found on tumblr, which was basically, 'Combine FMA and Big Hero 6'.

Please R&R:)

Edit: I appreciate your interest in this story, but it's marked Complete for a reason. This is a one-shot. And it's staying a one-shot because I don't have the time to continue it and I haven't seen the movie.