Omega Babysitter Izuku alpha Katsuki
I am not sure maybe a 3 or 4-year difference in age. Izuku is older and Katsuki has always said Izuku would be his bride kind of thing.
Izuku would watch Katsuki when their moms would go shopping trips.
Izuku has bite marks from Katsuki from when he was a kid the scars are still there since bites to the scent glands always scar. (Everyone brushed it off as Katsuki is going through a mine-mine-mine phase since he also kept biting on his mom and dad too and Inko but not as much as Izuku)
Izuku indulging in Katsuki's bad habits of scenting him since Katsuki has been doing it since he was little.
Katsuki tries to get away with taking baths with Izuku for as long as he can until his mom finally puts her foot down about it saying he is much too old now to be clinging on to Izuku the way he is.
Before Izuku's entrance exam for UA, he started working out because he heard rumors that there is a basic physical strength requirement for the Support and Management departments at UA and Izuku doesn't want to take any chances of not getting in. Katsuki was a brat about it secretly because soft-bodied omegas are best for having babies. And vocally he would complain working out is less time for Izuku to spend with Katsuki. Izuku doesn't relent so Katsuki starts doing his workouts with Izuku. But then Katsuki can't stop appreciating the body Izuku acquires from working out, especially his legs and thighs and other places.
Katsuki steals Izuku's clothes and rolls around on Izuku's bed to make sure it smells like him. He keeps Izuku's cloths for when he goes into rut.
Izuku demands hugs because he knows that older Katsuki won't ask for hugs anymore even if he secretly wants them.
Katsuki complains and pretends that it is a major inconvenience, but always squeezes him real tight and once his height is greater than Izuku's he tends to lift him up a little while hugging him.
Katsuki was short for an alpha up until the year before he starts going to UA. Some of his classmates tried (tried and failed horribly) to hold his height against him, but Katsuki wouldn't let something like that prevent him from being the top alpha at his school.
Izuku introduces Katsuki to Ochako(Alpha) and Iida(Beta) the summer that he passed his entrance exam. They are both surprised by Katsuki since Izuku always referred to him as his little guy and forever showed off pictures of them when they were younger that their mothers took. At that point, Katsuki would be taller than Izuku and would try to be as imposing as possible since these are the people that he keeps smelling on his precious Zuzu and a beta and an alpha seem like serious competition. The type of competition that isn't scared away by his bite marks and scent covering Izuku
Upperclassmen Izuku bringing Katsuki lunches to celebrate little accomplishments at UA cause he is so proud of his sweet little Kacchan or simply just because he wants to.
Izuku can cook really well, but isn't capable of doing beautiful presentation. Izuku brings Katsuki a bento and Kirishima makes the mistake of saying he understands Katsuki's devotion but he shouldn't make himself sick on poorly cooked food. Katsuki glares at him shoves a bit of something from the bento into his mouth and Kirishima is really surprised at how good it tastes even though it's a bit too spicy for him. Katsuki yells that he is never going to taste any more of Izuku's delicious cooking ever again! And starts grumbling about how presentation isn't important if it tastes like shit anyway. (I was thinking that it would be spicy chicken curry with root veggies and some bell peppers mixed in)
**I had it once at naruto ramen in New York it is delicious and spicy and definitely something Katsuki would enjoy when you get it really spicy it sort of looks toxic because of the spices and stuff but it tastes amazing!
Kirishima pokes fun at Katsuki having an older omega in his life
After having dealt with the people fawning over him from other classes.
Izuku still has no quirk and is attending UA for management. Toshinori has secretly been preparing Izuku for one for all Since Izuku is currently his temporary manager in training. But Toshinori still isn't sure about it yet even though Izuku's analytical skills are really good and he has the heart of a hero.
It could be that Toshinori feels bad because Izuku is an omega And Toshinori feel like he would be taking away opportunities for Izuku to have a family. (We will partially excuse this way of thinking since Toshinori is older and doesn't really know or understand omegas well since omegas being in heroing is a relatively new thing. And if he went to mostly heroing schools he would not have interacted with many and would base that sort of knowledge on stereotypes and Izuku's seemingly unending love of children)
Izuku living in the dorms but Katsuki constantly checks in because Izuku is an omega living alone. He would also bring stuff from Inko over to Izuku since Katsuki goes to UA.
(Excuse my terrible cheesy writing for a moment)
Izuku answering the door while in heat like an idiot
Katsuki forcing him back into bed and calling him an idiot for answering the door like that
What's wrong with you
I could have been anyone Deku!!
The getting overwhelmed by Izuku's scent
Realizing he is an idiot too
Izuku whining into his pillow telling Kacchan sorry
You smelled safe
Kacchan smells so nice
Katsuki crawls all over Izuku just like he used to when they would take naps together
Taking away the pillow and pulling Izuku into his arms tearing at his pajamas
Saying how Izuku smells good too! That, of course, Izuku should think that he smells good and that is it obviously because Izuku belongs to him just like he has always said
Katsuki is biting into Izuku's neck calling them kisses
Izuku starts grabbing at his back fingers catching on the fabric of his shirt
They aren't moving fast enough and Izuku's scent is getting stronger
They fall back into the mattress Katsuki's weight is warm and encompassing.
But he's pulling away his hands are gone. Izuku tries to keep him close tears on his cheeks
There is the sound of ripping fabric and cool air on his moist thighs.
Then Katsuki is back in his arms and Izuku feels full and there is burning and stinging that feels so good
Katsuki's thrusts are hard Izuku's too tight to go really fast and he can't stop biting on any bit of skin he can reach.
Izuku's mumbling and whining and he feels so good inside better than Katsuki dreamed about while fucking into his hand during his ruts. This is so much more than those shitty daydreams
Izuku is screaming and squeezing him real tight inside and with his arms, legs too.
There is a wetness on his stomach make the slid of their bodies against each other smooth and easy.
Izuku can't think there is too much to feel
Being full and empty, full and empty
Then Katsuki just keeps pushing in and it starts to feel like too much
His cock is rubbing around and the pressure on his insides is increasing it feels so good!
The bites on his neck feel sharp and bright. Izuku isn't sure he can handle much more the stretch is too much and he starts to scream again as he cums for the second time bearing down on Katsuki's knot. It's so painful.
Katsuki growls fuck repeatedly as his knot ties them together. They die down to a desperate sounding whine as he gives Izuku his warmth.
Katsuki rubs his face into Izuku's neck as he lifts their bodies to flip over
Izuku doesn't wake to the shifting of their bodies
Katsuki helps Izuku through his heat and Katsuki ends up in rut towards the end of Izuku's heat.