Author's notes: This is still just an idea, and battles and events will probably deviate from canon. Do give me your thoughts and feedback of the story so that I can improve! Thank you.
Fred and Wasabi leap into the air to grab the mask, but a wave of microbots surface to form a wall and effectively blocks them. Wasabi slices it in half with a triumphant crow, but Yokai only slams his fist on the ground without breaking a sweat. A grey dome readily encases him, with the threat of burying him alive. He knows Fred will jump into the scene and breathe fire to break his friend out; so Yokai lifts his hand and sends him flying with another turret of microbots.
GoGo – the only one who's presenting the slightest challenge, Yokai thinks – hurls her maglev discs at him, while Honey Lemon tries to trap and impede him with her chemical capsules. Their teamwork is sloppy, he thinks beneath his mask. Yokai simply ducks to avoid the discs, and swiftly conjures a u-shaped slide with his microbots to direct Honey Lemon's capsule back at her. Honey Lemon shrieks pathetically as it encases her feet in hardening foam, while GoGo grabs her discs back and narrowly avoids being punctured by more microbots.
Yokai stares at the chaos beneath him and smirks. They're all so pathetically weak, he sighs, trying to defeat him with skills they've barely grasped. It's nothing – nothing, he repeats fiercely – compared to the month of training he went through. It's entertaining to tear apart their confidence and will to fight. Only a few more attacks, a handful of close misses – maybe the capture or death of one of them – and everyone would crumple. But Yokai has to remember his other, grander plan, the one he can't afford to lose now. Tearing his eyes away from the fight raging below, he starts building a series of stairs with a lazy flick of his wrist and ascends slowly to the skylight.
On the ground, GoGo slides away, inches away from a whizzing microbot. Wasabi slices and dices, not daring to stop for fear that one second of uncertainty will leave him with a punctured heart. Fred jumps and somersaults ("Did you guys see that?") trying not to think too hard about what might have happened to him if he was even a second slower, and Honey Lemon slingshots and throws as she runs in dizzying circles.
It's all going according to plan, Hiro wants to assure them. They just have to continue distracting Yokai for a little while more while he and Baymax sneak up behind him to capture the mask. And as more microbots attacks start bursting out around them, Baymax navigates through the screaming bullets and veers faster, closer, so close that Hiro reaches out to grab the mask–
Only for Yokai to turn to look at him. Hiro freezes when their eyes meet for a second, then Baymax brakes mid-flight and he ends up catapulting right into Yokai instead.
Hiro lurches in surprise because he never miscalculates (Okay there was that one time but it wasn't exactly his fault). He can barely collect his thoughts properly before the ground rises up to meet him, and he and Yokai crash painfully into the concrete floor. Hiro scrabbles up, dazed and breathless. His fingers are clutching onto something and without thinking, Hiro raises his hand up.
It's Yokai's mask.
Hiro stares and stares and stares and thinks that this is the mask of Tadashi's murderer and he's finally done it and he looks up and–
It's Tadashi.
Hiro stares some more. Half of Tadashi's face is burnt and brutally scarred. It is a red leering mess that leaves his left eye nearly closed, and Hiro wonders if he can even see through it.
"Tada–"
"Give me that, you little brat!" Yokai – Tadashi – snaps at him and makes a grab for the mask. Hiro steps back instinctively, but Yokai only leaps forward and punches him in the gut, wrenching the mask free of his hands.
"Stupid kid," he mumbles, strapping the mask onto his forehead. He barely spares Hiro a glance before gathering his microbots again. They stream in like a million, skittering black spiders and Hiro wonders dazedly how his creation can look so horribly wrong.
But Tadashi is walking away and his mind clears in a panicky haste.
"Tadashi!" Hiro screams, while his mind whispers "Brother...?". Tadashi turns sharply once his name is called, and Hiro's heart lifts. It's him. It's him. It's him it's him it's him–
Then a fist connects with his stomach, and his line of repeated thoughts halt. The punch almost makes Hiro scream in pain, and he spits out flecks of blood and saliva as his knees start to give way. The next thing he knows, his head smacks into the wall and he is staring at his brother. He has his fingers wrapped around his neck in a frighteningly strong grip.
"Ta–"
"Quiet!" Tadashi spits. "I'm asking the questions–" Really, Hiro thinks, because, sure bro that is so original– "Who the hell are you, how do you know me, and why do you know my name?"
Hiro doesn't answer. He's too busy staring because how can Tadashi not know?
"Well?!" The fingers tighten and Hiro starts kicking his legs in discomfort.
"I'm Hiro! I'm your brother!" He wheezes. The fist only clenches harder and Hiro starts to panic.
"Liar." Tadashi says softly. His warm brown eyes are horrifyingly cold. "I have no family. I'm Tadashi." And that sentence is so ironic that Hiro would have laughed if he could stop choking.
"Really," Hiro gabbles as he spies Baymax in his periphery. "You're Tadashi Hamada. Eighteen this year. Studies – studied – at San Fransokyo–" His voice pitches a little higher when Tadashi's fingers twitches unconsciously. "Likes to wear cardigans and is actually a pretty cool older brother–" Tadashi's eyes widen and his fingers relax a fraction.
"Hiro?" He mumbles. Something – recognition, Hiro hopes – flashes in Tadashi's eyes and when he looks up, Hiro aches to call out his brother's name again–
"HIRO!" GoGo screams, and everything happens at once. Baymax's fist collides into Tadashi and sends him spiraling into a pile of rubble. Hiro drops to the ground and starts hacking violently. GoGo skids to a stop by his side with her maglev discs ready. Wasabi and Honey Lemon have their weapons poised to attack and Fred jumps into the scene (quite literally).
"We have him!" Fred crows, but when he sees the face that surfaces from the debris, his mouth drops open. Hiro sneaks a distracted glance at the team. GoGo's eyes are wide with shock, Honey's is brimming with tears, while Wasabi looks cautious and relieved and wary.
"Should have known you were lying," Tadashi spits at Hiro, his hand hovering over the scars on his face. "Trying to distract me – you're pretty sneaky for someone so young."
"I wasn't lying." Hiro's voice wobbles. "I am your brother–"
"Stop. Lying. LIKE. HIM." Tadashi shouts as he lashes out his left arm and thrusts his right fist forward. A string of microbots follow his motion and Hiro jumps to avoid the attack.
"Who?" He screams. "Tell me, Tada–" A microbot races past his eye as GoGo scoops him up and speeds away to safety. "GoGo, what are you doing? That's Tadashi–"
"That's a man trying to kill us." GoGo snaps, her hands clamping down on Hiro's shoulders firmly. "Tell me. If that really was Tadashi, why would he be doing this?" Hiro is speechless. He doesn't know. All he knows is that that's the face of his older brother and he wants to know what happened to him.
"We should at least knock him unconscious first before we decide whether he's really Tadashi," GoGo finally points out flatly. Hiro appreciates her level-headedness. He needs it in a time like this, when his thoughts are dangerously clouded and he doesn't think his brain can comprehend anything else. But at the same time, he thinks he can see a glimmer of worry in GoGo's eyes as she skates away.
"Get the mask," Hiro commands all of them as he jumps on Baymax. "Then hold it ransom and knock him out – or take him in, whichever works. We need to know if he's Tadashi or not." His grip on Baymax's armour tightens involuntarily as he prepares to take to the air, but the scene before his eyes stops him. He can hear Honey Lemon's gasp and GoGo's agitated growl.
Yokai has gathered all of his microbots again, and there's a split second where the entire army of it is suspended in the air – looming over them like the physical embodiment of death. Everything seems to play out in slow-motion; Hiro feels his heartbeat slow as he screams at everyone to get away, only for the microbots to fall at them like an unyielding tidal wave. Baymax flies higher to safety while Hiro shouts warnings, and he thinks that nothing, no training whatsoever, could have prepared them for this. Microbots and missiles whisper in his ear at every corner. Hiro orders Baymax to turn and spin and dodge but no matter where he goes, he is never that far away from a stray bullet that might hit him. They're inching closer to death at every move. All of them.
Suddenly, the microbots slam into Hiro's side without so much as a warning, and Hiro gasps in pain as he is sent tumbling from Baymax's back. His lips start to form a scream and he thinks this is it this is it this is it – until he crashes onto a platform of microbots and slides to the ground. Hiro jumps up in anxiety, but all he sees is Yokai's fleeting glance, before he disappears through the roof and everything goes black.