Yaku won against Kuroo in a match during junior high, causing Kuroo to hold a hold a grudge against him until they met again in their first year of high school. Kuroo thought of Yaku as the libero of the team that crushed his junior high dreams, while Yaku didn't even remember playing against Kuroo when asked about it.
The two joined the Nekoma volleyball team during their first year. They originally started off hating each other, mostly because of their clashing tastes and personalities. When asked what their goal for Nekoma was, they both answered with "to win nationals". Because of their common goal, the two chose to put their differences aside to play volleyball together.
Though Yaku and Kuroo are shown to still tease each other in their third year, they are on much friendlier terms and respect each other.
Although KuroYaku is not a very popular ship in the Haikyu!! fandom, it has a decent following. Most fans seem to have jumped onboard the ship after seeing the Nekoma Vs. Nohebi game. The anime's fourth season adapted this game, drawing more fans to Kuroo and Yaku's dynamic.
They are often portrayed together in fanworks with their friend, Nobuyuki Kai. They're referred to as the Nekoma Third-Years.
Most content with KuroYaku revolves around the two confessing to each other. The rest of the Nekoma volleyball team is usually included in these situations.
Yaku is often jokingly referred to as the team mom and Kuroo as the team dad.
On AO3, KuroYaku is the 9th most written ship for Kuroo and the 2nd most written ship for Yaku.
- Kuroo prefers fish, girls with long hair, sweet curry, going to the sea, dogs, Mikasa volleyballs, Nocchi, and Japanese-style. In contrast, Yaku likes meat, girls with short hair, spicy curry, going to the mountains, cats, Molten volleyballs, Kashiyuka, and Western-style.
- Kuroo calls Yaku by many nicknames, both to his face and to others. These include "Yakkun", "Yaku-paisen", and "Demon Senpai".