Because of Kenma's inability to make friends of his own accord, Kuroo, who was around his age, has always been his friend. Since they lived in the same neighbourhood, they would often play volleyball together. Under Kuroo's influence, Kenma joined the volleyball club in junior high and high school, and even talked him to not quit and stay in the club back in Kenma's first year. Kenma has also admitted the reason why he hasn't quit volleyball is because he would feel bad for letting Kuroo down. Kuroo is usually being seen looking after Kenma; he once brought Kenma back to the team when the latter got lost in Miyagi, and he made sure Kenma ate during the summer training camp. Kenma also calls Kuroo by his nickname "Kuro". In the Cross Team Match game, Kenma has stated that he had never failed a test because Kuroo teaches him before them.
The two started out in rough terms due to their completely different personalities but eventually became close and now comfortably rely on each other on the court. Yamamoto even asking Kenma to call him "Tora" instead. Yamamoto seems to also somewhat admire the fact that even though Kenma slacks off, and complains all the time he always sees things through to the end.