Misumi + Triangle Stickers
You know that Ultra Mr. Triangle Kazunari says he's designing for Misumi? He makes stickers.
(You can't tell me an art student with connections to a literal television crew doesn't have access to a sticker maker.)
Sumi has the BIGGEST grin when Kazu gives them to him. He is SO excited to take advantage of his new portable, stickable triangle friends.
He starts leaving them everywhere. Around the dorm, about the city, you name it.
He may go a bit overboard.
Often leaves them in places where he finds particularly good triangles, both as a reminder to himself and a general thank you to the universe. ("It's a triangle trade~!")
"What's with all the stickers?" he's eventually asked, after several are found around MANKAI. "If you find them, they bring you good luck! Everything's better with a triangle~!"
Believing this wholeheartedly, he makes special care to leave them in his favorite places, and places where people look down or in need of a good triangle. Including:
The briefcase of a sleeping man on the train, because he looks exhausted.
Inside the hood of a fussy baby's stroller while their mother is busy with another child. ("So they can always see it and not have to cry anymore~!")
Somehow manages to stick one on the triangle shaped part of the dorm roof, and if you look closely enough you can see it from the street looking down on you like a fair maiden on the front of a ship. (Just.... slightly more unsettling.)
The stage floor--Sakyo almost has a conniption. He can't get it off no matter what he tries, so he's eventually convinced to just leave it since it can't be seen from the audience anyway.
Tsuzuru finds one while riding the train one day and just lets out the biggest sigh.
If people he cares about are having a bad day he will go out of his way to sneak one into their belongings.
Taichi's having a rough time studying? Bam. There's a Sankaku-kun sticker inside his notebook.
Tenma's bonsai's not doing so well? Sankaku-kun, right on the pot. ("So it can grow big and strong and triangular~!" "That's not even how it works!")
Azuma's having nightmares? You bet your ass there's a Sankaku-kun sticker on his bunk, right there next to his pillow so he'll only have good dreams of triangles.
He wants to give one to Yuki while he's in the midst of Costume Crunch Hell but doesn't want to make him mad if he sticks it in a place he doesn't like and can't get if off, so he keeps the backing on one and slips it inside Yuki's sketchbook. (He'd never admit it, but Yuki is incredibly touched by this extra bit of thoughtfulness.)
Misumi also uses them as a form of thank you's and apologies, since he's not very good with words.
Kazunari has collected almost a dozen now for various reasons and he treasures every single one. His side of the room is starting to look almost as triangular as Misumi's.