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Izuku's always been crap at singing. In a world where your song is how you find your soulmate, Izuku found it disheartening at a young age that he couldn't carry a tune.
He knows Kacchan has a beautiful singing voice, though.
But he doesn't hear it again after their fall out as kids. To be honest, from the whispers Izuku hears from his middle school classmates, he thinks Katsuki never sings again after that. Just another thing about song that breaks Izuku's heart. So he pushes any thoughts about it away.
It's really when they both get into UA, and the school festival preparations start that Izuku thinks about it all again.
As a class, they've decided to put on a live band, and Izuku can't help the words that tumble from his mouth. "Kacchan has a beautiful singing voice."
The silence in the common room is deafening, and when Izuku finally notices it, he covers his mouth, but the damage is done. The silence breaks with so many questions at once that it makes Izuku's head spin.
"SHUDDUP!" Katsuki's booming voice put an end to the questions, except...
"Midoriya, c'mon, that voice can't possibly carry a tune."
Izuku gets angry on Katsuki's behalf. "You're wrong! It's the most beautiful thing I've ever heard, even though I only heard it once...!"
Katsuki's attention snaps to him, and Izuku wishes he'd kept his big mouth shut.
The whole room tenses as Katsuki grabs Izuku's arm; it feels like a brand against Izuku's skin, even though he knows Katsuki isn't using his quirk right now.
He lets Katsuki drag him away without a word of protest; their classmates are doing enough of that for him anyway.
Katsuki pays them no mind, dragging Izuku upstairs to his room. The trek sends Izuku's brain into overdrive as he tries really hard not to mutter his thoughts out loud as Katsuki stays deathly silent the whole way up.
Once they're inside Katsuki's room, everything changes.
Katsuki shoves Izuku, and he flails until he ends up in Katsuki's desk chair. "Listen up, nerd, and listen up good."
"Kacchan-"
"No, I talk; you listen. Shut the hell up."
Izuku's mouth click shut.
"I ain't singing for anyone. I don't sing for losers, and my soulmate is the biggest loser I know."
The words make Izuku freeze, and before thinking better of it, he needs to ask. "Kacchan knows his soulmate?"
Katsuki scowls. "Biggest. Idiot. I. Know," he says slowly.
Then he flicks Izuku's forehead painfully. "Ow, Kacchan!"
"Why do you think you're the only one who's ever heard me sing, Deku?"
Izuku stares at Katsuki before the words really sink in. "What? That can't be true. That would mean..."
"Fucking finally," Katsuki mutters.
"That can't be true! I can't sing Kacchan. You've never heard-"
"I have," Katsuki cuts him off. "And you're right, it's shit, on it's own. It happened before you got it into your head that your singing is god awful."
"Because it is!"
Izuku's protests go unheeded as Katsuki grabs his chin. "Sing with me, nerd. I'll show you why it isn't."
That's when Izuku's whole world turns upside down, and he hears Katsuki sing again. It's different from when they were children, but still so beautiful.
It's compelling him to reply, and Izuku swallows his fears, tentatively singing along. They blend in a beautiful melody Izuku didn't know he was capable of. It's entrancing, but when Katsuki stops, Izuku does as well.
"See? Fucking knew it."
Izuku blinks back tears, so overwhelmed by their shared song. "I didn't think... We're really soulmates? How could I sing so beautifully with you, Kacchan? I don't understand."
"We make each other better. That's all there is to it," Katsuki replies, pinching at Izuku's cheek. "Getting that through your thick skull?"
"Waah-chan," Izuku grumbles as he pushes away the offending hand. "...wait, did you always know? Did you think I rejected you, Kacchan!? I would never!"
Izuku can see Katsuki's face turning pink as he turns away.
"Shut up. I started to figure out that maybe that wasn't the case when you'd say your singing voice was shit. I knew that wasn't the case. Just needed to prove it to your ass."
Smiling softly, Izuku reaches out to grasp Katsuki's arm. "Consider me told," he murmurs then smiles widely. "So, now that I know... You should sing for the band!"
Katsuki spins around, looking like he's about to tear Izuku a new one when an idea dawns, and a maniacal grin forms on his face. "I'll do it if you sing with me."
Izuku tries to pull away, but Katsuki grabs his wrist.
"Wait, I couldn't! Ahhh, Kacchan, that's not fair!"
Katsuki laughs, pulling Izuku close as he struggles and whines. "No deal then."
"I wanna hear Kacchan sing on stage..."
"Only with you, nerd." Izuku feels his face flush as he gives up, hiding his face in Katsuki's chest.
"Didn't take you as a coward, Deku."
Even knowing it's a taunt, Izuku can feel Katsuki's words hitting their mark. "Everyone will know we're soulmates," he says, hoping that will deter Katsuki.
"They'll know we're more badass than them, so what?"
It's the final nail in his coffin. The idea of everyone knowing scares him, but it also makes his heart beat fast at the idea of the two of them being an unstoppable duo, in any context.
"...if the class is okay with it..."
Katsuki laughs. "I'll make the extras okay with it."