OMG... my MiziSua is actually NagiReo because oomf and I love Nagireo.. but ok let me put my thinking cap on and imagine Kurorin MiziSua.
THEY CATCH THE ROBOTIC FISHES... ? !? I love it when the bllk boys spend time together WGUHGH... I love doing mundane activities tgth type stuff... WAIT SO WHO IS THEIR IVANTILL. If we're stillg going by Till->Mizi and Ivan thinking him->Till is ~= Sua->Mizi originally until he realizes that MiziSua is reciprocated... it'd be really funny if it was also Kainess here like I personally think Ivan=Ness Till=Kaiser just because Black Sorrow is such a Ness song. So is Till the End but Black Sorrow is more on the nose with Ness. For krn & rn I can imagine Rin just like patting him on the head or something in silence under the stars <3 and then kurona jumps into the fxxking synthetic ocean and comes out with a fxxking robotic fish caught with his TEETH and Rin is like 'wh..what' (i love this man). Pre-Sae!mentaldestruction!Rin does vibe young Sua ... she looked happy too....

I'm pulling this out of my buttcrack but! Essay incoming because Kainess are my hyperfixation. Also for anyone who doesn't ship Kainess, these are noncanon hc's.. please don't m*rder me...
1. Both Ness and Kaiser are touch starved asf... Kaiser has that germaphobia clean freak thing going on in my mind so he doesn't let people come in contact with him & Ness has that childhood trauma leading to social aversion thing imo so the two of them if their relationship develops organically and not be so toxic doomed I think they'd find it hard to really be physically affectionate in the early stages. The fact Kaiser lets Ness into his personal space a lot lot more than anyone else (besides for soccer because obviously.. and even with Isagi he got his hands cleaned by Ness who had to touch his hands to do so) and in 243 even kicks his bare arse shows he's probably OK with physical contact with him
2. I feel like I can see Kaiser playing up his rivalry with Yoichi to make Ness feel jealous (not in canon, & I mean in private, saying things like 'Oh Yoichi wouldn't ___' to trigger him. Yess I'm thinking about that one Dan fanart. Ehem.) I was thinking about it but Ness feeling mad and hating on Isagi is quite abnormal, if you love someone why would you be jealous of someone that person hated and literally 'want to skin'? (Also, Ness has BPD headcanon intersectionality,) I feel like if Kaiser ever found out about Ness's jealousy over Yoichi in that regard (since in 207 Ness basically just acts like it doesn't bug him until after he leaves the viewing room THEN has THAT scene) that he'd poke fun at him for it or find it bewildering. He wouldn't #get_it.
3. Controversial opinion but they #switch. I will not elaborate because this is a pg15 space or something idk what kind of audience/extended friend group I'm fostering because I will continue shirtless kaiser posting. But my opinion comes from the fact that as an ace person it's really inconsequential to me...
4. This HC is a mixture of angst and fluff. Cliche cliche I know but I really am in support of the Kainess are childhood friends thing, I want both of them to have been like 14 in their trainee days. I just don't see like 16+ y/o Ness looking like that little itty bitty bite sized lil nugget & I can see Kaiser having gotten into lots of trouble as a kid (he def has mommy issues and no father) and he just got a tattoo severely underage. Sure maybe that doesn't count as childhood friends anymore but I'd rather they have met earlier because I really like the visceral feeling of having a longstanding, deep, twisted dynamic with someone where you both care about each other so much but also it's like you don't even know them at all. Ness I think erroneously believes that he knows everything there is to know about Kaiser (on the surface level) but subconsciously feels insecurity over how little he actually does. Like Ness was happy Kaiser shared things about himself to him so he's probably thinking 'oh he opened up to me/compared to everyone else, I know him the best' but is actually still terrified and anxiously attached, so this is my conclusion based on that. On Kaiser's side I think he has some doubt about Ness's feelings, and actually.. gasp? Loves him romantically more than Ness does Kaiser. Why? I feel that with Ness's upbringing he really just imprinted on Kaiser because no one ever treated him right his whole life. So Ness is more obsessed and reverent toward Kaiser than Kaiser is romantically inclined to Ness in any public manner, but Ness's extent of romantic feelings towards SPECIFICALLY Kaiser (like if anyone else saved him that day, then he'd be obsessed with them, too, it's not about Kaiser's inherent qualities) is not as much as that even. It's angsty. ANGST
5. HC, Ness is a P*tterhead and runs a book review blog and gets sent books by publishers to review and post about. I think Ness is one of those kids who COULD be book smart but has not too much, uh, as they put it, academic rigor, so he just fixates on other things? I think Kaiser jokes and judges him over it but then they do a movie marathon and he's converted until R*wling does something controversial for the 84783th time and then we all cry why can't we have girlbosses on this earth. The lord giveth and the lord taketh
6. They partake in old granny hobbies together like crochet. I think Kaiser likes coming off as suave and better than everyone else so wouldn't be caught having mundane hobbies like that, but Ness somehow (ok this is pre-Blue Lock pre-Kaiser's mental instability becoming so large he starts mistreating Ness) yaps about the wonders of needlefelting or something so much he tries it out of spite and then ends up liking it. They make matching accessories for each other and stuff. Kaiser's actually artistically inclined (an artisan! and also a theatre kid) so somehow his are better than Ness's which Ness is all >:( about jokingly
I wrote so much oh my god, I hope these are brainfood and not just weird yapping. Again if y'all disagree, that's O.K., I just personally like these and draw my Kainess with these kinds of things in the back burner of my brain going WRRR thank you for the lovely ask anon!