2(Haikyuu)

An Au where everyone is born with a daemon that represents your soulmate.

Kageyama was born with an adorable blonde rabbit that while shy when interacting with others that weren't Kageyama, she was the first to be attack or be in attack position when someone hurt or tried to hurt Kageyama.

He named the rabbit, Touka, and never left her behind even when he joined Kitagawa and went to training, Kageyama would take her with him and leave her in a bench to watch.

Kitagawa's volleyball team grew used to Touka and even started to bring their own daemons to see them and Touka interact which created some adorable moments like Touka being groomed by Iwazumi and Oikawa's daemons or even Kindaichi and Kunimi's daemon growling at anyone who suddenly appeared and scared her.

It made Kageyama's relationship with the team much better in canon as thanks to Touka he started to open himself up so what happened in third year or Oikawa's jealousy didn't happen(well, Oikawa would still be jealous but not enough to be blind that while Kageyama may be talented, he also trained hard as talent wasn't enough to go far in any sport).

So, when it came to deciding to which school they would go, Kageyama told them that Touka had seemed sure that he should go to Karasuno so he decided to do it as he may meet his soulmate if Touka's reaction mean anything.

Kitagawa's team supported him and they kept in contact so they were the first ones to receive a photo of Touka with a small black cat and of Kageyama with a blonde girl which they immediately knew was his soulmate.

Yes, Yachi is Kageyama's soulmate but if their relationship is romantic or platonic and anything else is up to the writer.