Some Seijoh headcanons because yes! 

CW: Mentions of divorced parents

o    There are a few rumours amongst the first years of the volleyball club (excluding Kindaichi and Kunimi) that Oikawa is secretly a descendant of one of the great Japanese emperors. It's not even remotely true but no one has bothered to tell the first-years that...

o    Iwaizumi is not photogenic. He almost always looks like he's about to sneeze.

o    Matsukawa is actually super laid-back and usually speaks with an apathetic tone of voice, unless he's making fun of people in a way that requires dramatics.

o    Hanamakki is really, really petty and he gets sulky/angry easily.

o    Yahaba, when fighting with Kyoutani, once threw a stick or ball and told him to 'go fetch'. Kyoutani punched him and they got into a really bad fight. Oikawa and Iwaizumi forced them to apologise. (pre-wall slamming scene)

o    Watari makes loads of jokes about having no friends and, every time he makes that joke in front of Yahaba, the setter threatens (very effectively) to ditch him and hang out with Oikawa.

o    Kyoutani's actually got a pretty good collection of simps in the second and first years but they're too intimidated to do anything about it.

o    Kindaichi still sometimes worries that he will never be a good enough player because of Kageyama's words in middle school. He practises super hard so he can be good enough for Oikawa and Yahaba.

o    Kunimi often plays things up so that his upperclassmen take him out to get ice cream or meat buns to make him feel better.

o    Kunimi often doesn't warm down properly and gets stiff joints because of it - Iwaizumi scolds him for not taking proper care of himself and Kindaichi always freaks out.

o    Kindaichi is not afraid of bugs and is one of the people who carefully and nicely takes bugs of any kind outside.

o    Kyoutani totally does the 'baby-talk' with dogs and all dogs just naturally adore him.

o    Watari actually did have a girlfriend from another school but, by the time he convinced Yahaba she was real and was going to introduce them, she broke up with him and he had to explain to a slightly amused and very cynical Yahaba that, yes, she did exist.

o    Yahaba is a little shit and enjoys starting rumours. They're mostly pretty harmless but hilarious.

o    Similarly to Tanaka, Hanamakki really enjoys it when his underclassmen call him 'senpai' because it makes him feel impressive and smart. Kunimi uses this to get free ice cream and meat buns.

o    Matsukawa is allergic to peanuts and uses it to debunk any and all 'deez nuts' jokes.

o    Iwaizumi is a certified Oikawa simp but the only difference is he usually isn't even aware that he's hardcore simping for him and has just accepted it as a part of himself.

o    Oikawa cuddles people in his sleep so, at training camps, the third years or Yahaba volunteer themselves to be cuddled to make him feel better. They volunteer because Oikawa always gives the best hugs but, usually, Iwaizumi or Yahaba win. (Oikawa cuddles in his sleep because it's leftover from the fear he has of being alone during and after his parents' divorce, because his sister had moved out he couldn't hug her anymore when he was upset)

o    Oikawa has once cursed the hell out of Ushijima in Spanish during the middle of a practise match at a training camp. Every team stopped to watch and he literally wouldn't shut up until Iwaizumi was forced to drag him away after fifteen minutes so he could serve.

o    Iwaizumi was (somehow) convinced that Asahi is a teacher substituting in the Karasuno Volleyball club. He was surprised when he found out Asahi was his age and nearly instantly asked him where he went to the gym.

o    Matsukawa's favourite class is maths, just so he can say he likes it and get the stunned reactions.

o    Hanamakki once threw up on Ushijima. It was completely by accident but, for the next week, Oikawa spoiled him with sweets, presents, compliments and random gifts of money for it.

o    Yahaba totally has a pash on Oikawa and looks up to him, pretty much desperate for his approval and for Oikawa to respect him as an equal. Kindaichi, Kunimi, Kageyama, Hinata and Atsumu would also all have pashes on him.

o    Watari has learnt as many swear words in other languages as he can.

o    When Kyoutani was about six or seven, he tried to wage a war against Ancient Egypt for worshipping cats over dogs.

o    Kindaichi doesn't regret his actions back in middle school, when he didn't hit one of Kageyama's tosses and get him off the court. Kageyama had been a terrible person and it was a singular act of rebellion. He was in the right. (Not the author being on Kindaichi's side in this conflict-)

o    Although Kunimi doesn't talk about it much, he hates Kageyama a little more than Kindaichi because I imagine there was a point where Kageyama wouldn't set for him because 'he never tried hard enough'.

o    Kunimi does not get along with teachers. They find his attitude unacceptable and he fins them invasive of his personal life.

o    Kindaichi has never gotten any detentions throughout his whole time at school.

o    Kyoutani pretends he's not scared of Oikawa but has a terrifying memory of one time where Oikawa screamed at him for never being there for the team and threatened him off the team because of how disrespectful he was being not just to his upperclassmen but also to the other first and second years. It was after that he stopped showing up to the club for a term, partly out of spite and mostly out of embarrassment and fear. (He secretly wonders and is scared of how Yahaba views him because of it because Oikawa was ruthless and did it in front of everyone.)

o    Watari has family in the United States and in Russia, so he's never around during the holidays because he's always abroad. He brings back loads of gifts for his friends.

o    Yahaba is the second-richest, after Oikawa. He also hates Shiratorizawa and Karasuno as a whole by default because he looks up to Oikawa so much.

o    Hanamakki doesn't have very good grades in things like science or maths but his grades in the creative subjects, like art and music, are really good.

o    Matsukawa once had a girl in the first-year ask him out. He found out later that she had mistaken him for Iwaizumi.

o    Iwaizumi goes easy on Oikawa during armwrestling matches because he doesn't want to hurt him. This usually makes him lose.

o    Oikawa is terrifying when he gets angry - he is completely willing to loudly lecture and threaten people in public, just to embarrass them, and he has even done this to Tendou before. Tendou is still scared of him today, despite pretending he isn't...