Deku didn't care about the consequences. All he knew was this instinctual desire to make this feisty Omega his. He'd known Kacchan for most of his childhood. When they had taken the test identifying their secondary genders, they'd gone together. They shared the results with each other. Kacchan had been so sure that he'd be an Alpha like Deku. The look of shock and disappointment on Kacchan's face as he read his results was only momentary. Even though he'd been officially identified as an Omega, he didn't let that sway him from his goals of becoming a top hero. When most Omegas became second-hand sidekicks at best, Kacchan fought to raise himself higher than that.
And this was something Deku found exhilarating.
He stayed close to his friend, encouraging him at each turn. Deku had earned his Omega friend's trust through the most vulnerable time in their lives. So when they graduated from UA High with full honors and with dozens of agencies fighting over them, it was Kacchan who decided for the both of them where they'd end up working. Deku knew that it didn't matter where they worked, as long as they were both in the same place. Keeping an eye on his Omega was more important than climbing higher on the hero status ladder.
Of course, Kacchan had never once considered Deku as a potential mate. He adhered to his strict medical regimen of suppressants and birth control to prevent any accidental heats from occurring. His focus was on his career as a hero. His primal desires were second to that cause.
And since Kacchan had never once doubted Deku's honesty, he didn't question it when Deku offered to pick up his suppressants while he was out in town. Deku knew that this was his opportunity to make a move that would change the rest of their lives.
So once he had Kacchan's pills in hand, Deku poured the contents into the toilet and flushed them away, refilling the bottle with an identical placebo he'd found after doing some research. Kacchan was none the wiser as he faithfully took his daily dose, washing it down with water and continuing his day. Now all Deku had to do was wait.
It didn't take long. Kacchan had delayed his first heat with suppressants for so long that when his body had expelled the last of it from his system, his hormones took over, seizing him in a fit of dizziness and nausea.
"Wha- What is this?!" Kacchan stuttered as he doubled over, his lower abdomen aching. His lower half trembled with a pain he didn't recognize. He'd only felt mere traces of this pain before. But his fear quickly settled once realizing what these symptoms meant.
Deku ran over as he saw Kacchan fall to his knees. "What's wrong, Kacchan, you don't look so good!" He slipped his arm beneath Kacchan's armpit, levering him up and supporting him with his shoulder. "Let's get you somewhere comfortable."
Back at Kacchan's apartment, he laid him gently down on his bed. Kacchan's breathing was ragged. Curled up in a fetal position, he hugged his knees against his chest, struggling to fight against the urges that plagued him.
"Kacchan, tell me if I'm wrong, but… Is this your first heat? The scent… You're giving off a strong scent, it's really hard to ignore."
He couldn't deny it, his lower body burned with the desire to be filled. The wetness that had gathered there soaked his underwear, making him uncomfortable as it clung to his clammy skin.
"I… don't know how it happened. I took my pills... Deku… It hurts… I don't know what to do…"
The ache in his genitals only grew as his cock swelled with anticipation. The presence of an Alpha made his hormones rage even harder. Deku's musky scent sent him reeling, even though he fought as hard as he could to avoid touching himself in front of his childhood friend, his hand reluctantly grasped his cock and started stroking it incessantly.
"Kacchan… I can't stand to see you in pain like this. Let me help you!" Deku invaded the space Kacchan had reserved between them. He was no longer in any condition to refuse. Kacchan impatiently wrapped his arms around his longtime friend, hoping that he wouldn't think any less of him once this moment had passed.
"Just make this go away, Deku. Make it stop hurting!"
Deku reached his hands down to Kacchan's pants, tugging and ripping them away at the seams. He'd waited too long to waste time caring about the mere fabric between them. Kacchan's cock shone against the light from the room, so wet and waiting for any kind of stimulus. Deku obliged and circled his fist around Kacchan's swollen erection, jerking his hand tightly against it while Kacchan squealed as his first foreign touch against his cock commenced.
"Easy, Deku… It's so… Ungh!" He couldn't help but thrust his hips towards Deku's grip, wanting to grind against the hand that held his cock firmly. He knew this wasn't as far as he wanted it to go though. Desires dampened his lower body, wishing for the Alpha to fill him.
"Kacchan… This won't be enough. You know that, right?"
He refused to answer, hoping that getting off with Deku's hand could be the end of this embarrassing interaction. But hope could only get him so far, instincts and nature just couldn't be ignored.
"Deku… Just this once. Fuck me. P-Please. It won't happen again, I swear."
Deku knew that this was it. He had Kacchan right where he needed him to be. Even though he'd betrayed his trust and given him fake suppressants, it didn't weigh down on his heart as he penetrated his best friend. The one that he'd been longing for since learning just what it meant to be Alpha or Omega. To him, this was only destiny in the making. All the time that they spent together was leading up to this moment. He was sure of that.
"Kacchan, you're so hot… I can't stop myself!" Deku cried out as he thrust into his soaking wet Omega's ass, his knot swelling and threatening to push itself across the edge of his rim.
"Wait… Deku!"
His cries were only half-hearted though. Kacchan wanted Deku's knot. Wanted to feel his seed flood into him and lock him in place, taking hold in his womb. Consequences be damned, he just needed to get fucked and filled like the slutty Omega he felt he was deep inside. If it had been anyone else, he wouldn't have let go so easily. But Deku… He knew the trust he had with him would transcend this momentary lapse of sanity. Deku was responsible, caring, and trustworthy.
So when Deku thrust his knot past Kacchan's sensitive entrance and poured his cum deep inside, Kacchan was caught unaware. Deku biting hard against the scent gland at the nape of his neck was of equal surprise to Kacchan. This wasn't just any reckless bite made in the throes of passion or ecstasy. He'd claimed him for his mate, without so much as conversing with him as to whether he'd accept him as such. The years he'd spent building himself up to be a top pro hero would be forfeit if word got out he was a mated Omega.
But for the moment, it didn't matter. Deku had fucked him senseless, spilled his seed inside and locked it within, making it nearly impossible to avoid impregnating him. He'd been faithful with taking his birth control, of course. But he'd also thought the same about his suppressants and look where that had gotten him. Could he be sure that this wouldn't result in him being filled with Deku's own pups?
"I-I'm sorry, Kacchan. I couldn't help myself," said Deku as his knot receded, finally breaking the hold between the two friends.
"Mmm… Just… Get something to wipe me off. I feel dirty," was all Kacchan could say. All he could do was hope and pray that even though his suppressants had failed him, his birth control wouldn't. He'd taken in so much of Deku's semen that it'd be nearly impossible for him to avoid pregnancy at this point. If only one mistake could be avoided, he needed it to be this one.
It was a good thing that Deku had switched out Kacchan's birth control at the same time as his suppressants. Now he knew that Kacchan would only be his. He'd marked him as his mate and filled him with his litter. Kacchan had no way of evading the life they were bound by fate to have together. Watching Kacchan clean himself after their tryst, Deku silently fantasized on how adorable their pups would be.