Characters relationships and how you met them:

(Twin) Brother: Tobio Kageyama
First bff: Satori Tendou (went to same same volleyball club in elementary school)
Second Bff‘s: Kuroo (grandmas neighbor) Kenma (through Kuroo), Bokuto, Akaashi (through Kuroo / volleyball club)
Close friend: Semi (through Tendou in middle school)
Bf: Koshi Sugawara
Later good friends: Yuu Nishinoya, Tsutomu Goshiki, Tadashi Yamaguchi

Storyline:

At 7 y/o you joined the volleyball team of a close neighborhood. During childhood you’ve often been alone, since your brother barely wanted to play with you. In that club you met Satori Tendou, who was two years older, yet it clicked immediately. You both had been outsiders most of your life, so you became friends, the best, from then on.
Later, when Tendou joined middle school he also became friends with Semi, who you’d get along well too, and you became sort of a trio
You had been a little of a problem child, at least according to your parents, which just blamed your for literally everything. So for the last two years of middle school, as much as parting from Tendou hurt, you moved in with your grandparents, who lived outside of Tokyo and which you loved dearly. You also knew the neighbor‘s kid, Kuroo and his best friend Kenma, which you’d befriend from now on and even would later join Nekoma and their volleyball club as their manager, as they had no female team. Tho you were allowed to train with them on regular days.
Through the volleyball club and Kuroo, you’d also get to know Bokuto and Akaashi from a nearby school. You’d also befriend them, and even hang out with them a lot during weekends, mostly as the group of five
During training camp you met Sugawara for the first time, who was nice to you from the beginning and functioned as sort of the bridge between you and Kageyama. Later, during the course of the first year of high school, when you moved back home, you two kept talking and supporting each other until something more developed.
After you came back and joined Karasuno high school in the middle of the first year, it was mostly Sugawara, Yamaguchi and Noya, who helped you settle in and so you befriended the two guys as well
Through practice of Semi and Tendou you’d also get to know Goshiki, another first year of their school. They would drag him to some hang outs of the three of you, so you could train and play two against two and so you’d become friends with Goshiki too.

Given Situation: Training camp. It was so damn hot - that’s just how Tokyo summers are. At least you weren’t the one playing. You were just filling up the water bottles for the Nekoma volleyball team. After all, you were their manager. Mainly because you were friends with Kuroo and Kenma, since Kuroo was the neighbor of your grandparents, where you lived, and so you grew up with them in a way. There were several reasons you moved in with your grandparents two years ago… and ever since then you haven’t really talked with your family at home. At least not with your twin brother… who you’d be seeing now. Tobio Kageyama. Now setter of Miyagi team Karasuno, which’d be joining the training camp this year. You were pretty damn nervous to meet him again. You were standing with Bokuto and Akaashi, also friends of yours and members of Fukurodani, when Karasuno arrived. You were a little confused when Tobio wasn’t there. Until you heard he’d be late. When he did arrive the tension between you was visible and soon Karasunos team found out you two were siblings. Damn, it was weird. And you were a little unsure how to act. Once it was dinner time, you were standing in queue for the buffet. Behind you, Karasuno‘s second setter. You hadn’t changed a single word with anyone from the team yet… but… there he stood, smiling friendly and beginning to talk calmly „So… you’re the girl everyone’s talking about now, hm? Don’t worry though, not in a bad way… uh… I’m Sugawara btw… sorry..“