I think I might come back now that I have, inspiration to draw or write for now ^^
So here is a bit info about Kageyama and Kato relationship, mostly their history throughout middle school, waring there is a bit of a spoiler about Kato's family in it so read it at your own risk OWO;.
Kageyama and Kato have been best friends since the start of there junior high day's. Those two meet during the volleyball tournament when Oikawa was provoking Kato and scaring her by saying she would be eaten alive by the other's the moment she step's foot on the court.
Knowing it was her first tournament and she was feeling anxious she quickly ran away in fear towards her upperclassmen, Kageyama sawed what happen but was called in by Iwaizumi that the there match was about to start. During the match's break, Kageyama asked Iwaizumi who was the girl that Oikawa was picking on, Iwaizumi revealed that she was just an old friend he and Oikawa grew up with
After the matches were over Kageyama decided to get a drink from one of the vending machines was out of the drink he wanted and realized, Kato had taken the last milk container. Kato realized Kageyama expression's and offered her drink to Kageyama cause his facial expression seemed as if he really wanted the drink.
Kageyama refuse her offer saying, that she got there first and she should take it, Kato insisted on Kageyama taking the drink anyway's because she sawed him play really well on the court and he didn't look or acted like a setter that would tournament people like Oikawa.
Kageyama was a taken by her word's and thanked her for the milk carton and asked if he could get a drink for her from the vending machines as a trade because he thought it was unfair he took her drink. Kato was about to reply to Kageyama only to be interpreted by Iwaizumi calling in Kageyama that the bus was leaving and congratulating on Kato at making it through her first tournament day since it was her first time participating in a tournament.
Ever since that day happened those two have became best friend's and started to hang out with each other, despite being from different school's. Those two were glad that neither of there school had to be rivals because Kato school didn't have a boy's team and Kageyama school didn't have a girl's team.
Those two would often casually hang out with each other and help each other out whether it was volleyball are school work. By the time there third-year approach Kato started to feel worried about Kageyama because his attitude had already destroyed his relationship with his teammates and she started to question herself maybe she should have told him what was going on by force rather than just trying to understand him and brush it off as if he was just having a bad day.
When asking him his sudden change, Kageyama only argued with her that it was nothing and that she should fouse on her skills because she seemed to be a bit weaker than him. All tho Kato admit that Kageyama was a better volleyball player her since he was a genius setter and she was just an ordinary setter who only gotten her skills of experience rather than talent.
When hearing that Kageyama got benched during the Junior High Athletics Meet, Kato got really concerned about him and confronted him about it face to face, Saying maybe he should apologize to his teammates that way they can put this ugly fight behind them.
Kageyama knew Kato was the only person who stayed beside him even after his sudden change yelled at her and said the most hurtful thing toward's her. Which was "Know I can see you why your father judge's you or despise you a bit", which was a metaphor for her that your the reason why your mom is dead.
Kato at first didn't believe what Kageyama said because she knew to talk about her mom was a sensitive issue for. but using it in a phrase like that caused her to get mad at him. Which resulted, her to slap Kageyama in the face and saying, "I thought you were my best friend...b-but maybe I should be blamed for my mom's death.....after a-all she did died while giving birth to me".
After that argument happened Kageyama began to hate himself, because not only did he got into a fight with his only best friend but he has lost her because oh his selfish anger, but he made her cry to the point where she told him that it would be best if they took a break from each other or never see each other again.