Tadashi found out when he was eight.
It was the first Christmas after his parents had died.
"Aunt Cass? Why are you - ?" Tadashi asked quietly, not sure what his Aunt is doing under the Christmas tree.
Aunt Cass whipped around. "Tadashi!" She froze, as if caught red-handed. Later, Tadashi reflected that that was in fact the case. "Why are you up?!"
Eying the armload of presents and noting the stocking stuffers, Tadashi came to a conclusion. "I heard something. But you should go to bed. Santa won't come unless you go to bed," Tadashi advised solemnly, at full volume.
"Shh!" Aunt Cass hissed. "You'll wake up Hiro!"
Eyebrows furrowed, Tadashi processed this information. Whispering, he asked, "So?"
Aunt Cass waved her hands at him. "Go to bed, Tadashi - Santa can't be seen by you!"
Tadashi's fast eight-year-old mind processed his aunt's words. "Aunt Cass..." he said in horror, "does Santa not go to homes with no parents?" It couldn't be - was Christmas ruined even more? Already he'd accepted that his parents wouldn't be there, but no presents from Santa was too much!
Aunt Cass's face fell at the sight of Tadashi's sudden gloom. "Tadashi, there's something I have to tell you..."
=====FIVE=====
Hiro was five.
It was the night before Christmas.
"Tadashi, Santa's coming tonight!" he exclaimed excitedly.
Tadashi grinned. "Oh? Did you send him your letter?"
Hiro pouted. "You know I did - you helped me address it, remember? I've never sent anything by international mail before."
"Oh yes, that's right." With all the patience of a caring older sibling, he waited for Hiro to continue.
"I mailed it yesterday. I hope he gets it in time."
Little did little Hiro know, but Tadashi had already relayed the contents of the letter to their aunt days ago, who was armed with just what Hiro wanted.
"I sure hope so."
Hiro smiled. "I'm sure he will! If he can fly around the world and give presents to everyone all in one night, then he can get my letter on the way too!"
Tadashi's heart stopped. He'd been hoping to keep up the Santa pretence until Hiro was at least a little older - he remembered the awe of Santa's magic from their parents, and he wanted Hiro to have at least a little bit of that, no matter how false.
Had Hiro figured it out?
But Hiro just kept smiling innocently, as if he had no idea.
"Sure thing, Hiro..."
=====SIX=====
Hiro was six.
It was the night before Christmas.
"Tadashi, look!"
Tadashi and Hiro were alone. Aunt Cass was busy in the cafe.
"What's up?"
"I wrote Santa a special letter!"
He proudly handed it to Tadashi, who read it through, and realized this might be more work than Aunt Cass wanted to put into pretending to be Santa. It was loaded with questions about Santa's work.
Question 1: How do you alter time?
There was a blank space following it, obviously expecting an answer.
"I want to know exactly how he does all of that magic," Hiro announced. "It's for science!"
Tadashi grinned, happy for his brother's excitement. "Let's leave it out for him by the fireplace."
The next day, Hiro was amazed. Santa had answered all his questions. He sat on Tadashi's lap, showing him the answers one by one and debating their merit.
"Look! He says he doesn't actually go down chimneys, he just teleports into the room but can't tell me how 'cause it's confidential magic!" He turned to Tadashi. "Does that mean he's working with the government?"
Tadashi shrugged. "I don't know - it's confidential, isn't it?"
Hiro furrowed his eyebrows. "Hm..." he hummed, pondering for just a second before reading the next answer.
"I asked him how come his handwriting looks like Aunt Cass's. He says he can copy anybody's handwriting with a special program that scans and moves a pen, since he can't be traced or else the grinch might find out his real identity." Hiro lit up. "I'm gonna make a program that copies handwriting!"
Aunt Cass frowned. She'd been listening absent-mindedly while making dinner. "Absolutely not, Hiro Hamada!"
Hiro squirmed a little bit, apparently deciding to let it go for now. Tadashi would have to watch for any writing robots. "And it says here that he knows when I'm sleeping with a long-distance health scanner, which also tells him if my neurotransmitters indicate that I feel guilt, which helps him find naughty kids." Hiro looked suitably impressed.
"And he makes his reindeer fly by feeding them... beans?" Hiro faltered. "Is that supposed to be a joke?"
Tadashi couldn't help laughing at Hiro's expression. But he good-naturedly smiled at him and urged Hiro to continue.
=====SEVEN=====
Hiro was seven.
It was late November.
"Tadashi! Tadashi!" Hiro looked terrified.
"What?!"
Fingers tapping against each other, Hiro looked around nervously. "Do you think Santa's watching?"
Tadashi paused. "I guess so."
Hiro paled. "Tadashi, I did something bad today."
"Oh?"
"Mr. Johnson, that mean teacher? - I hacked his mouse and made it work in reverse. He got mad and threw it against the wall - he'll never know it was me."
"Why'd you do that?"
"He said my parents didn't raise me right, that they didn't teach me to respect my elders. I just corrected him on a stupid math problem!"
With a sigh, Tadashi wordlessly pulled his little brother into a hug.
To his surprise, Hiro started crying after a few moments.
"Hiro?! Hey, what's up?"
"What if Santa gives me coal instead of a new microprocessor interfacer?!"
=====EIGHT=====
Hiro was eight.
It was the night before Christmas.
Tadashi felt some kind of satisfaction that he'd kept up the ruse so long. He wondered if Hiro would figure it out this year.
Apparently not.
"Look at this," Hiro whispered, so their aunt couldn't hear in the kitchen. "I'm going to get Santa on film, teleporting into our living room!"
Tadashi raised an eyebrow. "Oh?"
"Yea!" Hiro exclaimed. "I'll be able to track his face and everything! Come see what I've set up!"
Hiro had set up sonar equipment that would trip when Santa appeared in the room. It would trip a video camera, since Santa knew when naughty boys were watching and wouldn't come unless he knew that no one was actively watching.
Tadashi nodded and asked to see the code that tripped the camera.
The next morning, Hiro dejectedly informed Tadashi that somehow Mochi had set off the sensor, and the camera battery had died before Santa had come.
Tadashi smiled quietly.
=====NINE=====
Hiro was nine.
It was a week before Christmas.
Tadashi felt a sort of guilty satisfaction that Tadashi had beaten Hiro in terms of time to figure out Santa wasn't real, even it had only been a fluke for Tadashi.
"Okay, this year, I put pressure pads all over the floor under the carpet," he told Tadashi. "It'll trip an alarm that wakes me up, so I can catch him in the act and get him with this camera!" He held up a shiny digital camera that he'd gotten last year. "I've been testing it for a week, and I'll keep going right up until Christmas."
"Wow," said Tadashi. "That's impressive."
Somehow, the alarm went off only after Santa had come.
=====TEN=====
Hiro was ten.
It was the summer.
Hiro figured out how to make Mochi hoverboots and presumed that Santa must own a pair.
He consulted with Tadashi for a new Santa-trapping scheme, since a running camera would keep him from coming. They worked on a heat sensor together that fall. Laughter and voices were heard from the garage often that summer.
It didn't work.
=====ELEVEN=====
Hiro was eleven.
"Should we tell him Santa's not real?" Aunt Cass worried. Hiro was so smart, but he didn't seem to know about Santa, despite being an early high-schooler. He was busy writing yet another letter this year.
Tadashi shrugged. "I don't see any harm in keeping it up. He'll figure it out one way or another."
Aunt Cass smiled. "Yea, there's something about that adorable smile of his when he comes down Christmas morning, isn't there?"
=====TWELVE=====
Hiro was twelve.
It was early December.
"You sure you want to set traps? That's dangerous. What if Aunt Cass needs to use the bathroom? Or Mochi interferes again?"
"Oh... I guess I should try something else. Oo - how about I equip a stun gun under the cookies for Santa? No one will touch those but Santa. Help me set it up!"
Tadashi gingerly ate the cookies for Santa that year.
=====THIRTEEN=====
Hiro was thirteen.
He showed Tadashi his letter to Santa at his older brother's request.
This year, he wanted all digital things. A specific program, a digital file...
Basically, nothing in a box. And Santa didn't bring gifts to anyone but Hiro.
"I figured I'd save Santa a trip to our house this year."
"But Hiro," Tadashi asked, "aren't you going to try catching Santa again this year?"
Hiro looked up at his older brother, who had been busy with college recently and looked like he was in need of more sleep than there were hours in a day. Tadashi wondered if Hiro was trying to read his mind.
Hiro seemed to find whatever he was looking for. "I didn't want to bother you since you've been so busy," he said after a long moment of silence.
Tadashi laughed, secretly worried sick that his little brother wouldn't confide his Santa-catching in him. He would have no idea what Hiro was planning, if he was planning anything. "I've always got time to hear your Santa plans."
Hiro's smile quirked up, revealing the gap between his front teeth.
"I can try figuring out where the gifts came from online and trace it back to the source using these programs I've installed here..."
Tadashi breathed an internal sigh of relief.
Santa-catching plans were thwarted once again that year.
=====FOURTEEN=====
Hiro was fourteen.
It was late November.
Tadashi had passed away that fall.
Aunt Cass worried about how she was going to be Santa this year and if Hiro was too old. "Have you written your letter to Santa? Can I see it?"
Hiro smiled sadly and looked at Baymax, who was wearing a red Christmas hat, before turning to her. "It's okay, Aunt Cass. I'm not writing a letter this year."
"Why not, sweetie?"
Hiro looked away. "Because Santa's dead."
Baymax pulled Hiro close, and he didn't protest at Baymax's sudden hug for once. Aunt Cass looked down as she processed that Hiro had known about Santa. She wasn't sure what to say in her surprise and grief, which suddenly weighed on her more that she expected this time of year. It seemed that Hiro's innocence had died too.
After a long, long pause, she managed, "How long have you known?"
Hiro shrugged, face buried in Baymax's side. "Long enough. It just made Tadashi so happy," he whispered.
Aunt Cass's face crumpled. "Yea, it sure did." She sniffled and Baymax pulled her into his other arm's grasp.
"There, there," Baymax intoned peacefully, as he grabbed at the tissue box. "It will be all right."
=====FIFTEEN=====
Hiro was fifteen.
"I wrote a letter to Santa," he announced to Aunt Cass. "Can you mail it for me?"
"You wrote a letter to Santa?" she asked, surprised. Tadashi had stopped writing letters once he found out, opting to just tell her directly instead.
Hiro nodded and smiled sadly. "Yea. Santa's out there somewhere, isn't he? Maybe he'll get my message. There're a lot of things I want to tell him."
After Hiro left, Aunt Cass opened the letter Hiro wrote.
Dear Santa,
I wanted to save you a trip to my house this year, because I just want to spend Christmas with Aunt Cass - unless you're willing to get me that new Raspberry Pi computer chip. :-) I guess I won't try to catch you anymore, since my brother can't help me plan how to get you.
I guess I'll tell you about my aunt and me a bit. We've been doing better since the fire last fall. I'm having a great time at SFIT and...
Aunt Cass cried a little bit and smiled a little bit. Her tears blurred her vision.
Baymax was right; they would be all right.