Super short, just some self-indulgent comfort fluff
Tamaki has anxiety, that's obviously canon, but it's a bit of a step further in this oneshot. He has something similar to selective mutism, which I possibly have as well (hence the self-indulgent part). Basically when he gets overstimulated or anxious, there's a chance he'll go nonverbal, aka can't talk. It happens to me a lot after I get sensory overload or people yell or argue too much around me, a trait I projected onto Tamaki being around Bakugou.
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"Relax, sunshine, everything will be fine!" Mirio pat his boyfriend on the back gently, knowing he found comfort in his touch.
Today was supposed to be a fun training exercise for 1A, being pitted up against the triumvirate of UA's third years. There was only one small problem, Tamaki was beyond anxious. His midnight blue hair shielded his eyes as his finger nails dug into his palms, knuckles white and cresent dips forming in the outer layers of his skin from his finger nails.
His idiosyncrasies gave everything away, all the nervousness he was feeling. By the time the Big Three was outside the towering door of 1A, he could barely walk. He shuddered, taking a deep breath and following his friends into the classroom.
The class was rambunctious, a rowdy bunch of energetic teenagers willing to stop at nothing to get a few laughs and have fun, completely clashing with the ball-of-nerves's disposition. He felt tears pricking his eyes and froze in the doorway.
"I want to go home..." he whined, reaching a hand to grip the corner of his lover's shirt.
Mirio stopped, feeling the tug of the shorter boy's fist.
He patted his soft, shining dark hair and whispered a sweet "It'll be okay, I promise." He grinned, knowing his smile never failed to reassure Tamaki.
Those were the last words he spoke all day. All it took was a few sentences yelled louder than necessary and a couple of popping and crackling sparks from the class's resident walking explosion for the third year's anxiety to instantly sky rocket. He felt he was going to pass out right then and there. Luckily, he could get by with his friends doing his introductions for him, but eventually he'd have to do something other than stand completely still and panic.
His lips were glued shut, only leaving him able to make little whines and grunts to communicate. He stared, frozen, trying his best to keep himself breathing. Why did he have to panic so easily? Why did he have to go nonverbal at a time like this? He whined softly, a quiet noise going unnoticed by everyone except Mirio.
The blond shot Tamaki a sympathetic glance from the corner of his eye, using body language to try to convey reassurance, which of course the smaller didn't notice; he was too busy trying to remember what breathing felt like.
Time moved slower than a half-dead snail, but eventually they were all relocating to a sparing gym. As they walked, Mirio slipped his hand into Tamaki's, who's eyes shot to meet his.
"Hey, everything will be okay, I promise. The explosion guy isn't fighting, he's not allowed, so you don't have to worry about him. No one's going to hurt you, I'll be the only one fighting and you know you don't have to worry about me, I've got it under control!" He winked.
Tamaki only looked down to his feet as they walked, eyes glued to the tiled floor below.
"Are you verbal?" The taller of the two whispered.
Tamaki shook his head.
"Oh...," Mirio looked back ahead of them, watching where they walked for both himself and his boyfriend. "That's okay. Hey, when this is over, we can head back to the dorms. We can afford to skip just one half day, right? Come on, I'll take care of you, we can watch movies or something, or just sit together. How does that sound?"
Tamaki gazed up at Mirio's face, avoiding eye contact but still looking in the general area of his facial features. He nodded slightly in response, still not able to make his vocal cords cooperate.
"Unless you'd rather be alone? That's okay too!"
Tamaki shook his head no.
"Alright, then it's settled!" Mirio smiled, his pearly white teeth poking through his thin lips, "I've got you, okay, I'll do whatever it takes to make sure you're okay."
He really was like the sun.