When Kenma was younger, he was completely unable to make friends of his own accord. His only playmate back then was Kuroo, who was around his age and lived nearby. Kuroo introduced Kenma to volleyball sometime before they entered junior high; Kenma didn't like sports back then, but he found that volleyball was somewhat fun. Eventually, Kuroo convinced him to join the junior high volleyball club and Kenma received the number 4 on the team.
When he joined a volleyball team in high school, Kenma thought of quitting at first because of the pressure that the third years put on him. They often left him out specifically and made him run longer than the other members did. However, Kuroo convinced him otherwise, reminding Kenma of how amazing he is and that Kenma is now the official setter for the team.
Around his time as a first year, he also ran into Yamamoto and was quickly annoyed by his overzealous behavior and constant talk about "willpower". Yamamoto in return was annoyed at Kenma for being good despite his lack of passion or enthusiasm. At one point Kenma noticed Yamamoto was feeling down about a bad practice match they had where he hit a lot of "net outs" and due to Kuroo's advice about making more of an effort to get along with others, gave Yamamoto a piece of advice which then quickly turned to a clash of ideology, causing Kenma to yell for the first time in a while and ended with Fukunaga splashing water on the both of them to stop them from fighting.
Later on, Yamamoto realized that while Kenma is a "lazy sloth", he hates losing, and during a match against another school, was thankful that his analytical prowess was on Nekoma. Afterwards, Yamamoto told Kenma to call him "Tora" as a sign of friendship, which Kenma replied with "Do I have to?" but is what he calls him to this day.