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Izuku didn't think much of it at first. He was still getting used to having so many friends that the blush that followed the appearance of Kirishima's smile didn't surprise him.
There were different ones, Deku noticed, like the slightly crazed one full of teeth that made Kirishima's eyes sharp with danger. Or the one he gave to Bakugou, the corners of his eyes crinkled with the lift of his cheeks as he wrapped an arm around the blond's shoulders. The determined one that pushed everyone forward in training, a soundtrack of "so manly" echoing in their minds.
But the one that made him blush was the soft one. The one that was slightly crooked, that had red eyes shining in slight awe and respect and care. The one that always took Deku off guard, because teeth that sharp shouldn't look that sweet. But Izuku blushed often, especially when one of his friends complimented him, so he couldn't be flushed because of a smile. It had to be the kind words that joined it.
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The words were the worst. If he blushed from a smile then he burned from the words. Kiri did not hold back in his encouragement and support. Izuku knew this, but it didn't stop the blood from rushing to his face every time he heard the words "so manly" spoken to him. When accompanied with an honest fist pump and that devastating grin he stood no chance.
It was when they were both up late though that really made Izuku appreciate Kirishima's words, as well as his ears. Kiri actually listened to him. Every word. Deku was used to people stopping him from talking that it shocked him to finish a full word vomit without interruption. And when he was finished Kirishima just smiled and waved away his apology, continuing the conversation where he left off. If they both ignored how Izuku's eyes glistened then that was alright. Besides, he was flushing from the quiet words whispering out of the redheads mouth again.
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Next Izuku noticed the gentle touches. The hand on his shoulder always startled him so of course he'd notice. Deku used to jump whenever someone touched him from behind, now it was just a jerk of his head. The first time it happened Kirishima snatched his arm back as if it had been burned, but returned it after the many platitudes that spilled out of Deku's mouth. If he noticed how Deku slowly leaned into his body more he never said anything. But Izuku noticed. He noticed how Kirishima usually came from the front now. How he made sure to rub his head or pat his back when he passed by. Occasionally Kiri would play with his hair and he would turn into mush, incoherent mumbles escaping like the purring of a cat. Deku always found himself chasing after that hand with his head when he finished. Anything to bring back the soothing touch.
It didn't mean anything though. The way Izuku's heart beat faster when their arms brushed was nothing. He just wasn't used to all of this positive physical touch. He'd looked it up actually. The internet said he was probably touch starved. Either that, or he was dying. He'd much rather go with the former. It explained why he stood a little closer to Kirishima, why he leaned into his hand, why his heart beat faster every time Kiri was near. He just wanted to soak up all of those friendly gestures.
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When he found himself staring Izuku really should have gotten the hint. But Kirishima Eijiro had an amazing work ethic so of course it was hard to look away. He really wasn't watching the way sweat rolled down the muscles of his back. Honestly he was just cheering him on silently to not break his concentration.
Kirishima worked so hard, always pushing to do one more push-up, another mile, a last second toe touch. And maybe it's because he could see the disappointment when he didn't make it that he watched. The frustration of not doing better every day was so familiar that Izuku had to stop and look. He knew what it was like to dream about being a hero and not having the power to back it up.
So of course he watched. He watched as Kiri stood back up. He watched as his quirk exhausted him day in and day out. He watched as he sat through Kacchan's tutoring and studied so he could join the rest of the class when they trained. If he secretly hoped the other boy would take off his shirt like he usually did while training, well no one knew. And if his eyes wandered to follow the rolling muscles, well no one knew that either. He was just interested in watching Kirishima's quirk in use. That's all it was.
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It should have been obvious once he thought about it. All the signs were there. They had been there for quite some time now. But it took staring at his shoes to fully understand. It was when he was laying in the grass with one leg propped up on the other that he found himself fully focused on the color of his shoes. The red had always been a favorite, but it seemed extra cheerful that day. They were bright enough to bring a small smile to his face. That was then that Kirishima leaned over him, his face filling his entire view with red eyes and hair, causing his pulse to pick up. Izuku's smile grew as he thought that Kiri matched his red shoes, but Kirishima's eyes were much brighter than his shoes. He couldn't help but think Kiri looked pretty like that. Red hair against a blue sky, eyes bright with curiosity and a small smile that shone down on him. Yes, Kirishima was… Oh…
Oh.
That's what it was. Kirishima was pretty. That's why his heart beat faster and he craved his touch. That's why he stared and blushed. He liked Kiri. He cared for him as more than a friend.
He had a crush on Kirishima Eijiro.
Izuku blinked and quickly began overheating when he'd realized he'd zoned out causing Kiri to worry about him. As Kirishima turned to grab a water bottle and Deku wrapped his arms around his head he gave a small smile. If Izuku was going to have a crush, at least it was on someone as wonderful as Kirishima.