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Mitsuki and Inko introduce their little kids to each other for the first time and the moment Katsuki sees the other boy, half hiding behind his mother’s skirt because he’s a little bit shy (only at first) with wild green curls and big eyes, he thinks he’s the cutest thing he’s ever seen, but he doesn’t know how to handle all those feelings in his chest so he starts scowling at him.
“Katsuki, this is Izuku,” Inko says, gently pushing her son towards the other boy. Fortunately, Izuku seems immune to Katsuki’s aggressive expression and beams at him.
“Nice to meet you, Kacchan!”
“My name is not Kacchan, Deku!”
“Be nice, Katsuki!”
“Tch!”
However, the two boys get along immediately, Katsuki takes Izuku’s hand to show him around the children’s play area and tries to impress him, which works like a charm because in less than a second Izuku starts looking at him with star eyes and constantly praises him.
“It’s time to go, Katsuki!”
Much to Mitsuki’s confusion, Katsuki goes back to her with zero complaints and Izuku’s hand in his, looking like he doesn’t want to let go.
“You’ll like our home, Deku.”
Before Mitsuki can correct her son and tell him Izuku is not going to sleep over, little Katsuki turns around, looking very serious and determined before bowing in front of a confused, but amused Inko.
“Thanks. This wasn’t a shitty gift at all,” the blond kid says, pointing at Izuku. “This is maybe even a cool gift. I like him and I’ll keep him.”
Trying not to laugh, Inko covers her mouth as Mitsuki pinches her son’s cheek.
“He’s not a toy! You can’t own a person! Let him go!”
Katsuki refuses to do that and starts arguing with Mitsuki until Izuku sobs and tells him he wants to be with his own mom and Katsuki has to (reluctantly) let him go.
“But we can see each other another day, Kacchan!” Izuku promises after calming himself down.
“Tch! Whatever! Not that I care!”
He angry cries when he gets home and makes his parents promise not to tell ANYONE especially “shitty Deku” Masaru kindly asks what happened, but Katsuki stomps back to his room without saying anything else.
Then Mitsuki grins and whispers to her husband: “The kid has a crush. His first crush.”
Then later a very grumpy Katsuki plants himself in front of his parents while they’re trying to watch television.
“What is it, boy?”
“Do you need anything, my son?”
“What do I have to do to live with Deku?” He asks, narrowing his eyes, cheeks completely red.
“You can be his roommate, but when you two are older,” Masaru suggests.
“Do… roommates cuddle?”
“Yes, if they want to.”
Mitsuki hasn’t said a word because she’s trying to hold back a smirk.
“Do roommates… kiss?”
“See? I told you, Masaru!” Mitsuki chuckles. “No, boy. Better ask him to marry you instead.”
“But only after you two are of age,” Masaru kindly adds. “And you have to be sure that’s what he wants as well.”
“Why wouldn’t he want to marry me?”
“You have to make him fall in love with you first, Kats.”
“HA!” Katsuki looks back at his mother with so much confidence and determination it surprises both his parents. “I’ll make Deku love me in no time! And I’ll make him my wife so we can live together and kiss and cuddle!”
***
Much to Katsuki’s embarrassment, exactly 21 years later, that’s the story his mother shares with everyone on his wedding day with Izuku.
Izuku didn’t remember that moment very well so when his mother-in-law finishes the story he gets all excited and happy and starts tearing up before jumping into Katsuki’s arms and kissing him all over the face, just for that, Katsuki decides to forgive his mother just this once.