Assassination Classroom
So this is an idea where Karushuu are brothers and Karma's a year younger so he's a second year.
Karma is bored and is intrigued on why his father and older brother are so fixated and obsessed with 3-E so he skips school for a day.
Like, sure, he's always defended them, but he's never known the two only relevant males in his life to be so focused on them. He's confused on why Gakushuu would be so insistent that they have a secret.
So he goes and sees.
He watches as they do P.E. with Karasuma and how they train with knives and guns.
He sees a giant yellow octopus literally appearing and disappearing so fast.
Karma's surprised, but he doesn't let that faze him. He watches and sips his strawberry milk, then comes over.
He introduces himself as Akabane Karma (because they'll catch on if he introduces himself as Asano Karma, and because Akabane was his mother's maiden name) and says he's a new last-minute transfer student. The octopus and the military man (because exactly what else can he be, aside from many things) sigh and complain about not getting a notice beforehand, before welcoming him.
"Here, you will hone your assassination skills in order to kill me, as you've been informed. Otherwise, I'll just blow the Earth up like I did with the moon," the octopus says, and isn't that a surprise, like what. But Karma doesn't show any outward reaction. He just smiles. "I'm looking forward to it," he says, and he subtly swipes a knife and pellets and guns from different students. Luckily for him, someone had left a spare knife on the floor, beneath the tree he'd been hiding in, so he's stolen that too and cut it up before using some spare tape in his bag to tape them strips and pieces to his hand and palm. He reaches a hand out to shakes the octopus's tentacle hand, and it explodes, and Karma feels so much glee because he hadn't known a single thing about the octopus or the truth until today and he'd already damaged it with a minute-thought plan.
Later on in the day, Karma distracts the octopus and manages to damage it again with some carefully laid out BB pellets. He also manages to finish the quiz/test set out for him before exiting.
He next spends the rest of the day poking fun at the octopus and hanging out with the rest of 3-E, and he can see why his father and brother are so fixated and obsessed with them. He's starting to really like them a lot too.
"You guys are fun," he says. "And cool. But not as cool as my older brother."
Then the class starts pestering him about the older brother he mentioned, and he gives little details like he's smart, or he's talented, or he's popular. That's all they know.
Then the day is ruined when school ends and the chairman himself comes up to the Class 3-E building and sees Karma.
"Ah," the chairman goes. "What are you doing here? You haven't skipped class again, have you?"
And 3-E is confused, before Karma's like "class is boring and I wanted to see what got you guys so hooked on them" and Gakuhou goes "detention" and Karma goes "ugh you're no fun" and 3-E and the teachers are like "??????" before Gakuhou goes "I'm sorry for whatever trouble he's caused, I will make sure he will keep quiet about this matter" and when they're still confused, Gakuhou goes "Asano Karma is still a second year and should not have been skipping class to visit the E Class" and everyone's eyes bulge out and they're like "WHAT???!!!"
Gakuhou, Karma, 3-E, and the teachers have a long talk before matters are settled, and Karma promises to skip class and visit them again and Gakuhou's like "oh no you don't"
3-E and the teachers are left there in confusion as Karma walks off.
Karma meets up with Gakushuu, who scolds him for skipping class again, before Karma tells him of his day. Of course, he skipped out the classified stuff, but he described how much fun he had. "3-E's so cool!" he goes (with sparkling shining eyes and everything) "not as cool as you though" and Gakushuu feels much better and much more proud of himself after Karma says that last part.
Then the idea goes off the idea/imagination rails, and it tapers off into the unknown where nothing else pops up to pair with it.