Although Kazuki's face makes him appear zoned out or uninterested, he is actually quite the opposite. As a child, he almost always involved himself in Shōya's antics, as his daredevil mindset complimented Kazuki’s carefree yet enthusiastic attitude. Unlike his friend, he appeared to be a more responsible and hard-working student, attending cram school so he could succeed in middle school and beyond. He may also have been a member of the Boy Scouts, indicated by the text on the flag he carried around.

After helping pin Shōya as the sole bully of Shōko, the cold and condescending side of Kazuki began to show itself as he became Shōya’s primary bully, purposely doing things that he knew would make him miserable and feel isolated. This attitude would continue into middle school, where Kazuki went out of his way to tell people about Shōya’s actions.

I don't have a set voice for him, but please do make him sound like a male child!