even if seijoh!sugawara doesn't get a permanent spot in the starting lineup because of oikawa, he'd still play an active role in strategizing and making plays. oikawa is already a cunning setter on his own, but sugawara would amplify their team's offensive power with his tricks.

sugawara would always be the mediator when iwaizumi and oikawa argue because matsukawa and hanamaki just cackle in the background. this proves to be a great dynamic especially in team meetings when their precious captain and vice-captain butt heads.

that doesn't mean he's the gentle third-year though. he joins in when matsukawa and hanamaki play around, but he often gets away with it. he also teases oikawa a lot, and on the days he makes fun of iwaizumi, he doesn't get hit (much to the other setter's dismay).

kyoutani actually listens to him. the team understands why kyoutani listens to iwaizumi, but to sugawara? they don't really know. the aggressive second-year practically snarls at everyone during practice, but not to sugawara. never sugawara.

yahaba maybe never admit it out loud, but oikawa intimidates him. it's the reason why he doesn't ask for help from their captain unless oikawa is the one who offers himself. as a result, he chooses to seek guidance from sugawara instead.

(when sugawara realizes that yahaba only asks help from him because the younger setter is scared of oikawa sometimes, he tells oikawa this. yahaba would eventually notice that oikawa starts to give him more tips in practices even when he doesn't ask for them.)

considering that watari is proficient in setting as well, sugawara would often involve him in plays. most people would think seijoh only has one talented setter, but that's where they are mistaken. they have three.

oikawa is more than happy to be benched sometimes if it means seeing their opponents scramble in confusion once sugawara and watari pull off plays they would have never seen had it been oikawa on the court.

kindaichi and kunimi LOVE him. sure, they admire all their senpais in the team, but sugawara is their favorite. they never say this out loud, but they don't really need to. no one in the team is blind to how they glow when sugawara praises them.

sugawara would look really amazing in the aoba johsai uniform, and the teal of their jersey would suit him so well. this in itself is enough to push the seijoh!sugawara agenda.

addendum: sugawara has his own fan club because i said so. mizoguchi always sends iwaizumi off to look for their two third-year setters who probably got held up by their respective fan clubs.