When Mirio walked into class on the first day of second grade, there was a new boy there, the prettiest boy he'd ever seen.
The boy was nervous having everyone's eyes on him. He was painfully stuttering his way through his self-introduction, his eyes on the floor, only able to get out a handful of words.
But he was ethereal. He had thick black hair that fell into his face, and pointed ears that Mirio wanted to touch, and a magical wobbly smile when the teacher finally showed him mercy and let him take his seat.
Mirio bounced over to him the moment he was allowed to. "Did you mean to say 'heroes'?" he asked, with boundless eagerness. "I like heroes too!"
And the boy looked up, and met Mirio's eyes, and stole his heart.
A friendly smile, a shared dream, and soon Mirio and Tamaki were best friends. Actually, they were super best friends! At school, the two were inseparable, eating lunch together (away from all the other kids, who made Tamaki feel nervous), playing together at recess, and doing their best to always get paired up for assignments. Mirio adored Tamaki, and everyone knew it. He'd started to lose count of the number of times he'd been dared to tell Tamaki aishiteru.
After school, too, he spent his evenings with Tamaki as often as he could. They played heroes and villains outside in Mirio's backyard, and practiced their quirks together in Tamaki's tiny room. Mirio didn't have much control of his quirk yet, so he had a long way to go if he wanted to be a pro hero! But Tamaki helped with that. (Tamaki helped with a lot of things.)
Today he sat on the bed, opposite Tamaki, trying to phase his hand through Tamaki's. It was so hard, there were so many parts that all needed to work at the same time, and most of the time he just ended up smacking his hand into Tamaki's. Tamaki smiled and laughed at him-- he was working on manifesting cherry flowers from his other hand, with somewhat more success than Mirio. Every time he made one, he stuck it in Mirio's hair, which wasn't a great plan because Mirio kept accidently phasing his head, and they would slip out.
Eventually-- well, pretty quickly, actually-- Mirio's attention span ran out. "I can't do it anymore!" he said, falling back on the bed with a thump.
"You can't give up!" Tamaki cried, shaking his shoulders. "I can't be a hero if you don't become a hero too!"
"I'm only giving up for now," Mirio clarified. "We can practice together again tomorrow!" He sat back up, and slapped his hand against Tamaki's again, only this time without making any effort to phase his hand through. When their hands met, he took Tamaki's hand in his, and Tamaki squeezed back and blushed.
"Thanks for helping me with my quirk," Mirio said, quieter than he almost always was. Right now his heart was doing backflips in his chest, and he could barely look at Tamaki.
"It's no problem, I like helping you," Tamaki said with a smile.
In turn he got a beaming, gap-toothed smile of pure joy from Mirio. "You're such a good friend, Tamaki! My best friend!"
For the rest of the evening, they sat on Tamaki's bed and watched cartoons, and they didn't let go of each other's hands once.